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Cambridge Botanical Handbooks 

Edited by A. C. SEWARD and A. G. TANSLEY 



LICHENS 



CAMBRIDGE UNIVERSITY PRESS 

C. F. CLAY, MANAGER 
LONDON : FETTER LANE, E.C. 4 




LONDON : H. K. LEWIS AND CO., LTD., 

136, Gower Street, W.C. i 
LONDON : WHELDON & WESLEY, LTD., 

28, Essex Street, Strand, W.C. 2 
NEW YORK : THE MACMILLAN CO. 
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CALCUTTA I MACMILLAN AND CO., LTD. 
MADRAS J 
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TOKYO : MARUZEN-KABUSHIKI-KAISHA 



ALL RIGHTS RESERVED 



LICHENS 

BY 
ANNIE LORRAIN SMITH, F.L.S. 

ACTING ASSISTANT, BOTANICAL DEPARTMENT, BRITISH MUSEUM 



CAMBRIDGE: 

AT THE UNIVERSITY PRESS 

1921 

r r t\ ~J i 
JuUl 1 



IN RBAT MfTAUE. 




I O 



PREFACE 

THE publication of this volume has been delayed owing to war con- 
ditions, but the delay is the less to be regretted in that it has allowed 
the inclusion of recent work on the subject. Much of the subject-matter 
is of common knowledge to lichenologists, but in the co-ordination and 
arrangement of the facts the original papers are cited throughout. The 
method has somewhat burdened the pages with citations, but it is hoped 
that, as a book of reference, its value has been enhanced thereby. The 
Glossary includes terms used in lichenology, or those with a special licheno- 
logical meaning. The Bibliography refers only to works consulted in the 
preparation of this volume. To save space, etc., the titles of books and papers 
quoted in the text are generally translated and curtailed: full citations-will 
be found in the Bibliography. Subject-matter has been omitted from the 
index : references of importance will be found in the Table of Contents or 
in the Glossary. 

I would record my thanks to those who have generously helped me 
during the preparation of the volume : to Lady Muriel Percy for taking 
notes of spore production, and to Dr Cavers for the loan of reprints. Prof. 
Potter and Dr Somerville Hastings placed at my disposal their photographs 
of the living plants. Free use has been made of published text-figures 
which are duly acknowledged. 

I have throughout had the inestimable advantage of being able to consult 
freely the library and herbarium of the British Museum, and have thus been 
able to verify references to plants as well as to literature. A special debt 
of gratitude is due to my colleagues Mr Gepp and Mr Ramsbottom for 
their unfailing assistance and advice. 

A. L. S. 

LONDON, 

February ) 1920 



CONTENTS 

PAGE 

GLOSSARY xix 

ERRATA xxii 

INTRODUCTION xxiii 

CHAPTER I 
HISTORY OF LICHENOLOGY 

A. INTRODUCTORY i 

B. PERIOD I. PREVIOUS TO 1694 2 

C. PERIOD II. 16941729 5 

D. PERIOD III. 1729 1780 6 

E. PERIOD IV. 17801803 ...... 9 

F. PERIOD V. 18031846 ...... 10 

G. PERIOD VI. 18461867 . '. ^15 

H. PERIOD VII. 1867 AND AFTER '18 

CHAPTER II 
CONSTITUENTS OF THE LICHEN THALLUS 

I. LICHEN GONIDIA 
i. GONIDIA IN RELATION TO THE THALLUS 

A. HISTORICAL ACCOUNT OF LICHEN GONIDIA . . 21 

B. GONIDIA CONTRASTED WITH ALGAE 22 

C. CULTUREEXPERIMENTSWITHTHELlCHENTHALLUS 24 

D. THEORIES AS TO THE ORIGIN OF GONIDIA . . 25 

E. MICROGONIDIA 26 

F. COMPOSITE NATURE OF THALLUS .... 27 

G. SYNTHETIC CULTURES 27 

H. HYMENIAL GONIDIA 30 

I. NATURE OF ASSOCIATION BETWEEN ALGA AND 

FUNGUS 31 

a. Consortium and symbiosis 

b. Different forms of association 

J. RECENT VIEWS ON SYMBIOSIS AND PARASITISM . 36 

2. PHYSIOLOGY OF THE SYMBIONTS 

A. NUTRITION OF LICHEN ALGAE 39 

a. Character of algal cells 

b. Supply of nitrogen 

c. Effect on the alga 

d. Supply of carbon 

e. Nutrition within the symbiotic plant 
/ Affinities of lichen gonidia 

B. NUTRITION OF LICHEN FUNGI 44 

C. SYMBIOSIS OF OTHER PLANTS . . . . . 45 



viii CONTENTS 

II. LICHEN HYPHAE 

PAGE 

A. ORIGIN OF HYPHAE . 46 

B. DEVELOPMENT OF LICHENOID HYPHAE . . . 47 

C. CULTURE OF HYPHAE WITHOUT GONIDIA . . 49 

D. CONTINUITY OF PROTOPLASM IN HYPHAL CKLLS . 51 

III. LICHEN ALGAE 

A. TYPES OF ALGAE 51 

a. Myxophyceae associated with Phycolichens 

b. Chlorophyceae associated with Archilichens 

B. CHANGES INDUCED IN THE ALGA .... 60 

a. Myxophyceae 

b. Chlorophyceae 

C. CONSTANCY OF ALGAL CONSTITUENTS ... 63 

D. DISPLACEMENT OF ALGAE WITHIN THE THALLUS . 64 

a. Normal displacement 

b. Local displacement 

E. NON-GONIDIAL ORGANISMS ASSOCIATED WITH 
LICHEN HYPHAE . . ... . . . 65 

F. PARASITISM OF ALGAE ON LICHENS . . , . 65 

CHAPTER III 
MORPHOLOGY 

I. GENERAL ACCOUNT OF LICHEN STRUCTURE 
ORIGIN OF LICHEN STRUCTURES 

A. FORMS OF CELL-STRUCTURE . . '.' . 67 

B. TYPES OF THALLUS . . . \ ,. . . 68 

a. Endogenous thallus 

b. Exogenous thallus 

II. STRATOSE THALLUS 
i. CRUSTACEOUS LICHENS 

A. GENERAL STRUCTURE . ... .. - -..', . 70 

B. SAXICOLOUS LICHENS . . . - .. > . ; 

a. Epilithic lichens 

aa. Hypothallus or protothallus 
bb. Formation of crustaceous tissues 
cc. Formation of areolae 

b. Endolithic lichens 

c. Chemical nature of the substratum 

C. CORTICOLOUS LICHENS 

a. Epiphloeodal lichens 

b. Hypophloeodal lichens 



CONTENTS ix 

2. SQUAMULOSE LICHENS 

I'AGE 

A. DEVELOPMENT OF THE SQUAMULE .... 79 

B. TISSUES OF SQUAMULOSE THALLUS . . . . 81 

3. FOLIOSE LICHENS 

A. DEVELOPMENT OF FOLIOSE THALLUS ... 82 

B. CORTICAL TISSUES 82 

a. Types of cortical structure 

b. Origin of variation in cortical structure 

c. Loss and renewal of cortex 

d. Cortical hairs 

C. GONIDIAL TISSUES . . . . . . . 87 

D. MEDULLA AND LOWER CORTEX .... 88 

a. Medulla 

b. Lower cortex 

c. Hypothallic structures 

E. STRUCTURES FOR PROTECTION AND ATTACHMENT . 91 

a. Cilia 

b. Rhizinae 

c. Haptera 

F. STRENGTHENING TISSUES OF STRATOSE LICHENS . 95 

a. Produced by development of cortex 

b. Produced by development of veins or nerves 

III. RADIATE THALLUS 

1. CHARACTERS OF RADIATE THALLUS 

2. INTERMEDIATE TYPES OF THALLUS 

3. FRUTICOSE AND FILAMENTOUS THALLUS 

A. GENERAL STRUCTURE OF THALLUS 101 

Cortical Structures 

a. The fastigiate cortex 

b. The fibrous cortex 

B. SPECIAL STRENGTHENING STRUCTURES . . .103 

a. Sclerotic strands 
/'. Chondroid axis 

C. SURVEY OF MECHANICAL TISSUES .... 105 

D. RETICULATE FRONDS 106 

E. ROOTING BASE IN FRUTICOSE LICHENS . . . 108 

IV. STRATOSE-RADIATE THALLUS 
i. STRATOSE OR PRIMARY THALLUS 

A. GENERAL CHARACTERISTICS HI 

B. TISSUES OF PRIMARY THALLUS 112 

a. Cortical tissue 

b. Gonidial tissue 

c. Medullary tissue 

d. Soredia 



x CONTENTS 

2. RADIATE OR SECONDARY THALLUS 

PAGE 

A. ORIGIN OF THE PODETIUM . . . .114 

B STRUCTURE OF THE PODETIUM . . . .114 

a. General structure 

b. Gonidial tissue 

c. Cortical tissue 

d. Sored ia 

C. DEVELOPMENT OF THE SCYPHUS . . . .117 

a. From abortive apothecia 

b. From polytomous branching 

c. From arrested growth 

d. Gonidia of the scyphus 

e. Species without scyphi 

D. BRANCHING OF THE PODETIUM . . . . 119 

E. PERFORATIONS AND RETICULATION OF THE PODE- 
TIUM 120 

F. ROOTING STRUCTURES OF CLADONIAE . . .121 

G. HAPTERA 122 

H. MORPHOLOGY OF THE PODETIUM . . . .122 
I. PILOPHORUS AND STEREOCAULON . . . .125 



V. STRUCTURES PECULIAR TO LICHENS 

i. AERATION STRUCTURES 

A. CYPHELLAE AND PSEUDOCYPHELLAE . . .126 

a. Historical 

b. Development of cyphellae 

c. Pseudocyphellae 

d. Occurrence and distribution 

B. BREATHING-PORES 129 

a. Definite breathing-pores 

b. Other openings in the thallus 

C. GENERAL AERATION OF THE THALLUS . . .132 

2. CEPHALODIA 

A. HISTORICAL AND DESCRIPTIVE 133 

B. CLASSIFICATION 135 

I. CEPHALODIA VERA 

II. PSEUDOCEPHALODIA 

C. ALGAE THAT FORM CEPHALODIA . . . .136 

D. DEVELOPMENT OF CEPHALODIA . . . 137 

a. Ectotrophic 

b. Endotrophic 

c. Pseudocephalodia 

E. AUTOSYMBIOTIC CEPHALODIA 140 



CONTENTS xi 

3. SOREDIA 

PAGE 

A. STRUCTURE AND ORIGIN OF SOREDIA . . .141 

a. Scattered soredia 

b. Isidial soredia 

c. Soredia as buds 

B. SORALIA 144 

<7. Form and occurrence of soralia 

b. Position of soraliferous lobes 

c. Deep-seated soralia 

C. DISPERSAL AND GERMINATION OF SOREDIA . . 147 

D. EVOLUTION OF SOREDIA 148 

4. ISIDIA 

A. FORM AND STRUCTURE OF ISIDIA . . . -149 

B. ORIGIN AND FUNCTION OF ISIDIA . . . . 151 

VI. HYMENOLICHENS 

A. AFFINITY WITH OTHER PLANTS . . . . 152 

B. STRUCTURE OF THALLUS 153 

C. SPORIFEROUS TISSUES 154 



CHAPTER IV 

REPRODUCTION 

I. REPRODUCTION BY ASCOSPORES 

A. HISTORICAL SURVEY 155 

B FORMS OF REPRODUCTIVE ORGANS . . . .156 

a. Apothecia 

b. Perithecia 

C. DEVELOPMENT OF REPRODUCTIVE ORGANS . . 159 

i. DISCOLICHENS 

a. Carpogonia of gelatinous lichens 

b. Carpogonia of non-gelatinous lichens 

c. General summary 

d. Hypothecium and paraphyses 

e. Variations in apothecial development 

aa. Parmeliae 
bb. Pertusariae 
cc. Graphideae 
dd. Cladoniae 



xii CONTENTS 

2. PYRENOLICHENS 

PAGE 

a. Development of the perithecium 

b. Formation of carpogonia 

D. APOGAMOUS REPRODUCTION 174 

E. DISCUSSION OF LICHEN REPRODUCTION . . . 177 

a. The Trichogyne 

b. The Ascogonium 

F. FINAL STAGES OF APOTHECIAL DEVELOPMENT . 181 

a. Open or closed apothecia 

b. Emergence of ascocarp 

G. LICHEN ASCI AND SPORES 184 

a. Historical 

b. Development of the ascus 

c. Development of the spores 

d. Spore germination 

e. Multinucleate spores 
/ Polaribilocular spores 

II. SECONDARY SPORES 

A. REPRODUCTION BY OIDIA 189 

B. REPRODUCTION BY CONIDIA 190 

a. Rare instances of conidial formation 

b. Comparison with Hyphomycetes 

C. CAMPYLIDIUM AND ORTHIDIUM .... 191 

III. SPERMOGONIA OR PYCNIDIA 

A. HISTORICAL ACCOUNT OF SPERMOGONIA. . . 192 

B. SPERMOGONIA AS MALE ORGANS .... 193 

C. OCCURRENCE AND DISTRIBUTION .... 193 

a. Relation to thallus and apothecia 

b. Form and size 

c. Colour 

D. STRUCTURE 196 

a. Origin and growth 

b. Form and types of spermatiophores 

c. Periphyses and sterile filaments 

E. SPERMATIA OR PYCNIDIOSPORES .... 201 

a. Origin and form 

b. Size and structure 

c. Germination 

d. Variation in pycnidia 

F. PYCNIDIA WITH MACROSPORES 204 



CONTENTS xiif 

PAGE 

G. GENERAL SURVEY ..... . 205 

a. Sexual or asexual 

b. Comparison with fungi 
' c. Influence of symbiosis 

d. Value in diagnosis 



CHAPTER V 
PHYSIOLOGY 

I. CELLS AND CELL PRODUCTS 

A. CELL-MEMBRANES 209 

a. Chitin 

b. Lichenin and allied carbohydrates 

c. Cellulose 

B. CONTENTS AND PRODUCTS OF THE FUNGAL CELLS. 213 

a. Cell-substances 

b. Calcium oxalate 

c. Importance of calcium oxalate 

C. OIL-CELLS 215 

a. Oil-cells of endolithic lichens 

b. Oil-cells of epilithic lichens 

c. Significance of oil-formation 

D. LICHEN-ACIDS 221 

a. Historical 

b. Occurrence and examination of acids 

c. Character of acids 

d. Causes of variation in quantity and quality 

e. Distribution of acids 

E. CHEMICAL GROUPING OF ACIDS 225 

I. ACIDS OF THE FAT SERIES 

II. ACIDS OF THE BENZOLE SERIES 

SUBSERIES I. ORCINE DERIVATIVES 
SUBSERIES II. ANTHRACENE DERIVATIVES 

F. CHEMICAL REAGENTS AS TESTS FOR LICHENS . 228 

G. CHEMICAL REACTIONS IN NATURE .... 229 

II. GENERAL NUTRITION 

A. ABSORPTION OF WATER 229 

a. Gelatinous lichens 

b. Crustaceous lichens 

c. Foliose lichens 

d. Fruticose lichens 

B. STORAGE OF WATER 232 

b 



xiv CONTENTS 

PAGE 

C. SUPPLY OF INORGANIC FOOD . . . .... . 232 

a. In foliose and fruticose lichens 

b. In crustaceous lichens 

D. SUPPLY OF ORGANIC FOOD . ... - '. . . 235 

a. From the substratum 

b. From other lichens 

c. From other vegetation 

III. ASSIMILATION AND RESPIRATION 

A. INFLUENCE OF TEMPERATURE ., .. . . . 238 

a. High temperature 

b. Low temperature 

B. INFLUENCE OF MOISTURE . . . . . . 239 

a. On vital functions 

b. On general development 

IV. ILLUMINATION OF LICHENS 

A. EFFECT OF LIGHT ON THE THALLUS . . . 240 

a. Sun lichens 

b. Colour-changes due to light 

c. Shade lichens 

d. Varying shade conditions 

B. EFFECT OF LIGHT ON REPRODUCTIVE ORGANS . 244 

a. Position and orientation of fruits with regard to light 

b. Influence of light on colour of fruits 

V. COLOUR OF LICHENS 

A. ORIGIN OF LICHEN-COLOURING . V.' . 245 

a. Colour given by the algal constituent 
A Colour due to lichen-acids 

c. Colour due to amorphous substances 

d. Enumeration of amorphous pigments 

e. Colour due to infiltration 



CHAPTER VI 
BIONOMICS 

A. GROWTH AND DURATION OF LICHENS . . .252 

B. SEASON OF FRUIT FORMATION. . .255 

C. DISPERSAL AND INCREASE 256 

a. Dispersal of crustaceous lichens 

b. Dispersal of foliose lichens 

i. Dispersal of fruticose lichens 

D. ERRATIC LICHENS . . . ... . . , 258 



CONTENTS xv 

PAGE 

E. PARASITISM 260 

a. General statement 

b. Antagonistic symbiosis 

c. Parasymbiosis 

d. Parasymbiosis of fungi 

e. Fungi parasitic on lichens 

/ Mycetozoa parasitic on lichens 

F. DISEASES OF LICHENS 268 

a. Caused by parasitism 

b. Caused by crowding 

c. Caused by adverse conditions 

G. HARMFUL EFFECT OF LICHENS .... 269 
H. GALL-FORMATION 270 



CHAPTER VII 

PHYLOGENY 
I. GENERAL STATEMENT 

A. ORIGIN OF LICHENS . 272 

B. ALGAL ANCESTORS ....... 273 

C. FUNGAL ANCESTORS 273 

a. Basidiolichens 

b. Ascolichens 

II. THE REPRODUCTIVE ORGANS 

A. THEORIES OF DESCENT IN ASCOLICHENS . . 273 

B. RELATION OF LICHENS TO FUNGI .... 275 

a. Pyrenocarpineae 

b. Coniocarpineae 

c. Graphidineae 

d. Cyclocarpineae 

III. THE THALLUS 

A. GENERAL OUTLINE OF DEVELOPMENT OF THALLUS 281 

a. Preliminary considerations 

b. Course of evolution in Hymenolichens 

c. Course of evolution in Ascolichens 

B. COMPARATIVE ANTIQUITY OF ALGAL SYMBIONTS . 282 

C. EVOLUTION OF PHYCOLICHENS 283 

a. Gioeolichens 

b. Ephebaceae and Collemaceae 

c. Pyremdiaceae 

d. Heppiaceae and Pannariaceae 

e. Peltigeraceae and Stictaceae 



xvi CONTENTS 

PAGE 

D. EVOLUTION OF ARCHILICHENS . . . . 287 

a. Thallus of Pyrenocarpineae 

b. Thallus of Coniocarpineae 

c. Thallus of Graphidineae 

d. Thallus of Cyclocarpineae 

AA. LECIDEALES 

aa. COENOGONIACEAE 

bb. LECIDEACEAE AND GYROPHORACEAE 
cc. CI.ADONIACEAE 

i. Origin of Cladonia 

i. Evolution of the primary thnllus 

3. Evolution of the secondary thallus 

4. Course of podetial development 

5. Variation in Cladonia 

6. Causes of variation, 

7. Podetial development and spore-dissemination 

8. Pilophorus, Stereocaulon and Argopsis 
BB. LECANORALES 

aa. COURSE OF DEVELOPMENT 
bb. LECANORACEAE 
cc, PARMELIACEAE 

dd. USNEACEAE 

ee. PHYSCIACEAE 



CHAPTER VIII 

SYSTEMATIC 
I. CLASSIFICATION 

A. WORK OF SUCCESSIVE SYSTEMATISTS . . . 304 

a. Dillenius and Linnaeus 

b. Acharius 

c. Schaerer 

d. Massalongo and Koerber 

e. Nylander 

/ M tiller- Argau 

g. Reinke 

h. Zahlbruckner 

B. FAMILIES AND GENERA OF ASCOLICHENS . . 311 
C HYMENOLICHENS 342 

II. NUMBER AND DISTRIBUTION 

i. ESTIMATES OF NUMBER 
2. GEOGRAPHICAL DISTRIBUTION 

A. GENERAL SURVEY 343 

B. LICHENS OF POLAR REGIONS 345 

C. LICHENS OF THE TEMPERATE ZONES . . .348 

D. LICHENS OF TROPICAL REGIONS .... 352 

III. FOSSIL LICHENS 



CONTENTS xvii 

CHAPTER IX 
ECOLOGY 

PAGE 

A. GENERAL INTRODUCTION .* 356 

B. EXTERNAL INFLUENCES . . . " . . . 357 

a. Temperature 

b. Humidity 

c. Wind 

d. Human Agency 

C. LICHEN COMMUNITIES 362 

1. ARBOREAL 363 

a. Epiphyllous 

b. Corticolous 

c. Lignicolous 

2. TERRICOLOUS 367 

a. On calcareous soil 

b. On siliceous soil 

c. On bricks 

d. On humus 

e. On peaty soil 
/ On mosses 

g. On fungi 

3. SAXICOLOUS 371 

a. Characters of mineral substrata 

b. Colonization on rocks 

c. Calcicolous 

d. Silicicolous 

4 . OMNICOLOUS LICHENS 376 

5. LOCALIZED COMMUNITIES 378 

a. Maritime lichens 

b. Sand-dune lichens 

c. Mountain lichens 

d. Tundra lichens 

e. Desert lichens 

f. Aquatic lichens 

D. LICHENS AS PIONEERS . .... 392 

a. Soil-formers 

b. Outposts of vegetation 

CHAPTER X 
ECONOMIC AND TECHNICAL 

A. LICHENS AS FOOD 395 

a. Food for insects 

b. Insect mimicry of lichens 

c. Food for the higher animals 

d. Food for man 



CONTENTS 



B. LICHENS AS MEDICINE ...... 45 

a. Ancient remedies 

b. Doctrine of "signatures" 

c. Cure for hydrophobia 

d. Popular remedies 

C. LICHENS AS POISONS ..... . . 410 

D. LICHENS USED IN TANNING, BREWING AND DISTIL- 
LING ....... . . .411 

E. DYEING PROPERTIES OF LICHENS . . . .411 

a. Lichens as dye-plants 

b. The orchil lichen, Roccella 

c. Purple dyes : orchil, cudbear and litmus 

d. Other orchil lichens 

e. Preparation of orchil 

f. Brown and yellow dyes 

g. Collecting of dye-lichens 

h. Lichen colours and spectrum characters 

F. LICHENS IN PERFUMERY ...... 418 

a. Lichens as perfumes 

b. Lichens as hair-powder 

G. SOME MINOR USES OF LICHENS .... 420 

APPENDIX ....... .- . 421 

ADDENDUM ...... , 422 

BIBLIOGRAPHY . . . ... ... 423 

INDEX . . . . . . ... 448 



GLOSSARY 

Acrogenous, borne at the tips of hyphae; see spermatium, 312. 

Allelositismus, Norman's term to describ* the thallus of Moriolaceae (mutualism), 313. 

Amorphous cortex, formed of indistinct hyphae with thickened walls ; cf. decomposed 

cortex. 

Amphithecium, thalline margin of the apothecium, 157. 

Antagonistic symbiosis, hurtful parasitism of one lichen on another, 261 et seq. 
Apothecium, open or disc-shaped fructification, 11, 156 et passim. Veiled apothecium, 

169. Closed or open at first, 182. 
Archilichens, lichens in which the gonidia are bright green (Chlorophyceae), 52, 55 

et passim. 

Ardella, the small spot-like apothecium of Arthoniaceae, 158. 
Areola (areolate), small space marked out by lines or chinks on the surface of the thallus, 

73 et passim. 

Arthrosterigma, septate tissue-like sterigma (spermatiophore), 197. 
Ascogonium, the cell or cells that produce ascogenous hyphae, 180 et seq. 
Ascolichens, lichens in which the fungus is an Ascomycete, 159, 173 et passim. 
Ascus, enlarged cell in which a definite number of spores (usually 8) are developed ; cf. 

theca, 157, 184. 
Ascyphous, podetia without scyphi, i \qetpassim. 

Biatorine, apothecia that are soft or waxy, and often brightly coloured, as in Biatora, 158. 

Blasteniospore, see polarilocular spore. 

Byssoid, slender, thread-like, as in the old genus Byssus.. 

Campylidium, supposed new type of fructification in lichens, 191. 

Capitulum, the globose apical apothecium of Coniocarpineae ; cf. mazaedium, 319. 

Carpogonium, primordial stage of fructification, 160, 164 et passim. 

Cephalodium, irregular outgrowth from the thallus enclosing mostly blue-green algae ; or 

intruded packet of algae within the thallus, 1 1, 133 et passim. 
Chrondroid, hard and tough like cartilage, a term applied to strengthening strands of 

hyphae, 104, 1 14. 

Chroolepoid, like the genus Chroolepis (Trentepohlia). 
Chrysogonidia, yellow algal cells ( Trentepohlia). 

Cilium, hair-like outgrowth from surface or margin of thallus, or margin of apothecium, 91. 
Consortium (consortism), mutual association of fungus and alga (Reinke); also termed 

"mutualism," 31, 313. 

Corticolous, living on the bark of trees, 363. 
Crustaceous, crust-like closely adhering thallus, 70-79. 
Cyphella, minute cup-like depression on the under surface of the thallus (Sticta, etc.), 

i.i, 126. 

Decomposed, term applied to cortex formed of gelatinous indistinct hyphae (amorphous), 

73-8i et passim, 357. 

Determinate, thallus with a definite outline, 72. 
Dimidiate, term applied to the perithecium, when the outer wall covers only the upper 

portion, 159. 



xx GLOSSARY 

Discoid, disc-like, an open rounded apothecium, 1 56. 

Discolichens, in which the fructification is an apothecium, 160 et seq. 

Dual hypothesis, the theory of two organisms present in the lichen thallus, 27 et seq. 

Effigurate, having a distinct form or figure; cf. placodioid, 80, 201 

Endobasidial, Steiner's term for sporophore with a secondary sporiferous branch, 200. 

Endogenous, produced internally, as spores in an ascus, 179; see also under thallus. 

Endolithic, embedded in the rock, 75. 

Endosaprophytism, term used by Elenkin for destruction of the algal contents by enzymes 

of the fungus, 36. 

Entire, term applied to the perithecium when completely surrounded by an outer wall, 159. 
Epilithic, growing on the rock surface, 70. 
Epiphloeodal, thallus growing on the surface of the bark, 77. 
Epiphyllous, growing on leaves, 363. 
Epithecium, upper layer of thecium (hymenium), 158. 
Erratic lichens, unattached and drifting, 259. 

Exobasidial, Steiner's term for sporophore without a secondary sporiferous branch, 200. 
Exogenous, produced externally, as spores on tips of hyphae ; see also under thallus. 

Fastigiate cortex, formed of clustered parallel hyphal branches vertical to long axis of 

thallus, 82. 

Fat-cells, specialized hyphal cells containing fat or oil, 75, 215 et passim. 
Fibrous cortex, formed of hyphae parallel with long axis of thallus, 82. 
Filamentous, slender thallus with radiate structure, 101 et seq. 
Foliose, lichens with a leafy form and stratose in structure, 82-97. 
Foveolae, Foveolate, pitted, 373. 

Fruticose, upright or pendulous thallus, with radiate structure, 101 et seq. 
Fulcrum, term used by Steiner for sporophore, 200. 

Gloeolichens, lichens in which the gonidia are Gloeocapsa or Chroococcus, 284, 373, 389. 

Gonidium, the algal constituent of the lichen thallus, 20-45 et passim. 

Gonimium, blue-green algal cell (Myxophyceae), constituent of the lichen thallus, 52. 

Goniocysts, nests of gonidia in Moriolaceae, 313. 

Gyrose, curved backward and forward, furrowed fruit of Gyrophora, 184. 

Hapteron, aerial organ of attachment, 94, 122. 

Haustorium, outgrowth or branch of a hypha serving as an organ of suction, 32. 

Helotism, state of servitude, term used to denote the relation of alga to fungus in 

lichen organization, 38, 40. 
Heteromerous, fungal and algal constituents of the thallus in definite strata, 13, 68, 305 

et passim. 

Hold-fast, rooting organ of thallus, 109, 122 et passim. 
Homobium, interdependent association of fungus and alga, 31 
Homoiomerous, fungal and algal constituents more or less mixed in the thallus, 1 3, 68, 

305 et passim. 

Hymenial gonidia, algal cells in the hymenium, 30, 314, 315, 327. 
Hymenium, apothecial tissue consisting of asci and paraphyses ; cf. thecium, 157. 
HymenolichenSj, lichens of which the fungal constituent is a Hymenomycete, 152-154, 342. 
Hypophloeodal, thallus growing within the bark, 78, 364. 
Hypothallus, first growth of hyphae (proto- or pro-thallus) persisting as hyphal growth at 

base or margin of the thallus, 70, 257 et passim. 
Hypothecium, layer below the thecium (hymenium), 157. 



GLOSSARY xxi 

Intricate cortex, composed of hyphae densely interwoven but not coalescent, 83. 
Isidium, coral-like outgrowth on the lichen thallus, 149-151. 

Lecanorine, apothecium with a thalline margin as in Lecanora, 158. 

Lecideine, apothecium usually dark-coloured or carbonaceous and without a thalline 

margin, 158. 

Leprose, mealy or scurfy, like the old form genera, Lepra, Lepraria, 191. 
Lichen-acids, organic acids peculiar to lichens, 221 et seq. 
Lignicolous, living on wood or trees, 366. 
Lirella, long narrow apothecium of Graphideae, 158. 

Mazaedium, fructification of Coniocarpineae, the spores lying as a powdery mass in the 

capitulum, 176. 

Medulla, the loose hyphal layer in the interior of the thallus, 88 et passim. 
Meristematic, term applied by Wainio to growing hyphae, 48. 

Microgonidia, term applied by Minks to minute greenish bodies in lichen hyphae, 26. 
Multi-septate, term applied to spores with numerous transverse septa, 316 et seq. 
Murali-divided, Muriform, term applied to spores divided like the masonry of a wall, 187. 

Oidium, reproductive cell formed by the breaking up of the hyphae, 189. 

Oil-cell, hyphal cell containing fat globules, 215. 

Orculiform, see polarilocular. 

Orthidium, supposed new type of fructification in lichens, 192. 

Palisade-cells, the terminal cells of the hyphae forming the fastigiate cortex, 82, 83. 

Panniform, having a felted or matted appearance, 260. 

Paraphysis, sterile filament in the hymenium, 157. 

Parasymbiosis, associated harmless but not mutually useful growth of two organisms, 263. 

Parathecium, hyphal layer round the apothecium, 157. 

Peltate, term applied to orbicular and horizontal apothecia in the form of a shield, 336. 

Perithecium, roundish fructification usually with an apical opening (ostiole) containing 

ascospores, 158 et passim. 

Pervious, referring to scyphi with an opening at the base (Perviae\ 118. 
Phycolichens, lichens in which the gonidia are blue-green (Myxophyceae), 52 et passim. 
Placodioid, thallus with a squamulose determinate outline, generally orbicular ; cf. 

effigurate, 80. 

Placodiomorph, see polarilocular. 

Plectenchyma (Plectenchymatous), pseudoparenchyma of fungi and lichens, 66 et passim. 
Pleurogenous, borne laterally on hyphal cells ; see spermatium, 312. 
Pluri -septate, term applied to spores with several transverse septa, 321 et seq. 
Podetium, stalk-like secondary thallus of Cladoniaceae, 1 14, 293 et seq. 
Polarilocular, Polaribilocular, two-celled spores with thick median wall traversed by a 

connecting tube, 188, 340-341. 

Poly torn ous, arising of several branches of the podetium from one level, 118. 
Proper margin, the hyphal margin surrounding the apothecium, 157. 
Prothallus, Protothallus, first stages of hyphal growth ; cf. hypothallus, 71. 
Pycnidiospores, stylospores borne in pycnidia, 198 et passim. 
Pycnidium, roundish fructification, usually with an opening at the apex, containing 

sporophores and stylospores ; cf. spermogonium, 192 et seq. 
Pyrenolichens, in which the fructification is a closed perithecium, 173 et passim. 

Radiate thallus, the tissues radiate from a centre, 98 et seq. 



xxii GLOSSARY 

Rhagadiose, deeply chinked, 74 ; cf. rimose. 
Rhizina, attaching "rootlet," 92-94. 
Rimose, Rimulose, cleft or chinked into areolae, 73. 
Rimose-diffract, widely cracked or chinked, 74. 

Scutellate, shaped like a -platter, 156. 

Scyphus, cup-like dilatation of the podetium, in, 117. 

Signature, a term in ancient medicine to signify the resemblance of a plant to any part 

of the human body, 406, 409. 

Soralium, group of soredia surrounded by a definite margin, 144. 
Soredium, minute separable particle arising from the gonidial tissue of the thallus, and 

consisting of algae and hyphae, 141. 
Spermatium, spore-like body borne in the spermogonium, regarded as a non-motile male 

cell or as a pycnidiospore, 201. 

Spermogonium, roundish closed receptacle containing spermatia, 192. 
Sphaeroid-cell, swollen hyphal cell, containing fat globules, 215. 
Squamule, a small thalline lobe or scale, 74 et passim. 
Sterigma, Nylander's term for the spermatiophore, 197. 
Stratose thallus, where the tissues are in horizontal layers, 70. 
Stratum, a layer of tissue in the thallus, 70. 
Symbiont, one of two dissimilar organisms living together, 32. 
Symbiosis, a living together of dissimilar organisms, also termed commensalism, 31, 32 

et seq. 

Tegulicolous, living on tiles, 369. 

Terebrator, boring apparatus, term used by Lindau for the lichen " trichogyne," 179. 

Thalline margin, an apothecial margin formed of and usually coloured like the thallus ; 

cf. amphithecium. 
Thallus, vegetative body or soma of the lichen plant, 11,421. Endogenous thallus in which 

the alga predominates, 68. Exogenous thallus in which the fungus predominates, 69. 
Theca, enlarged cell containing spores ; cf. ascus. 
Thecium, layer of tissue in the apothecium consisting of asci and paraphyses ; cf. 

hymenium, 157. 
Trichogyne, prolongation of the egg-cell in Florideae which acts as a receptive tube ; 

septate hypha in lichens arising from the ascogonium, 160, 177-181, 273. 

Woronin's hypha, a coiled hypha occurring in the centre of the fruit primordium, 1 59, 163. 



ERRATA 



p. 24. For Baranetsky razaTB 

p. 277. For Ascolium read Acolium. 

p. 318. For Lepolichen coccophora read coccophorus. 



INTRODUCTION 

LICHENS are, with few exceptions, perennial aerial plants of somewhat 
lowly organization. In the form of spreading encrustations, horizontal leafy 
expansions, of upright strap-shaped fronds or of pendulous filaments, they 
take possession of the tree-trunks, palings, walls, rocks or even soil that 
afford them a suitable and stable foot-hold. The vegetative body, or thallus, 
which may be extremely long-lived, is of varying colour, white, yellow, 
brown, grey or black. The great majority of lichens are Ascolichens and 
reproduction is by ascospores produced in open or closed fruits (apothecia 
or perithecia) which often differ in colour from the thallus. There are a few 
Hymenolichens which form basidiospores. Vegetative reproduction by 
soredia is frequent. 

Lichens abound everywhere, from the sea-shore to the tops of high 
mountains, where indeed the covering of perpetual snow is the only barrier 
to their advance ; but owing to their slow growth and long duration, they 
are more seriously affected than are the higher plants by chemical or other 
atmospheric impurities and they are killed out by the smoke of large towns: 
only a few species are able to persist in somewhat depauperate form in or 
near the great centres of population or of industry. 

The distinguishing feature of lichens is their composite nature: they 
consist of two distinct and dissimilar organisms, a fungus and an alga, which, 
in the lichen thallus, are associated in some kind of symbiotic union, each 
symbiont contributing in varying degree to the common support : it is 
a more or less unique and not unsuccessful venture in plant-life. The 
algae Chlorophyceae or Myxophyceae that become lichen symbionts or 
"gonidia" are of simple structure, and, in a free condition, are generally to 
be found in or near localities that are also the customary habitats of lichens. 
The fungus is the predominant partner in the alliance as it forms the fruiting 
bodies. It belongs to the Ascomycetes 1 , except in a few tropical lichens 
(Hymenolichens), in which the fungus is a Basidiomycete. These two 
types of plants (algae and fungi) belonging severally to many different 
genera and species have developed in their associated life this new lichen 
organism, different from themselves as well as from all other plants, not 
only morphologically but physiologically. Thus there has arisen a distinct 
class, with families, genera and species, which through all their varying forms 
retain the characteristics peculiar to lichens. 

1 E. Acton (1909) has described a primitive lichen Rotrydina vnlgaris, in which there is no 
fruiting stage, and in which the fungus seems to show affinity with a Hyphomycete. 



xxiv INTRODUCTION 

In the absence of any " visible " seed, there was much speculation in 
early days as to the genesis of all the lower plants and many opinions 
were hazarded as to their origin. Luyken 1 , for instance, thought that lichens 
were compounded of air and moisture. Hornschuch 2 traced their origin to 
a vegetable infusorium, Monas Lens, which became transformed to green 
matter and was further developed by the continued action of light and air, 
not only to lichens, but to algae and mosses, the type of plant finally evolved 
being determined by the varying atmospheric influences along with the 
chemical nature of the substratum. An account 3 is published of Nees von 
Esenbeck, on a botanical excursion, pointing out to his students the green 
substance, Lepraria botryoides, which covered the lower reaches of walls and 
rocks, while higher up it assumed the grey lichen hue. This afforded him 
sufficient proof that the green matter in that dry situation changed to 
lichens, just as in water it changed to algae. An adverse criticism by 
Dillenius 4 on a description of a lichen fructification is not inappropriate to 
those early theorists : " Ex quo apparet, quantum videre possint homines, 
si imaginatione polleant." 

A constant subject of speculation and of controversy was the origin of 
the green cells, so dissimilar to the general texture of the thallus. It was 
thought finally to have been established beyond dispute that they were 
formed directly from the colourless hyphae and, as a corollary, Protococcus 
and other algal cells living in the open were considered to be escaped 
gonidia or, as Wallroth 5 termed them, " unfortunate brood-cells," his view 
being that they were the reproductive organs of the lichen plant that had 
failed to develop. 

It was a step forward in the right direction when lichens were regarded 
as transformed algae, among others by Agardh 6 , who believed that he had 
followed the change from Nostoc lichenoides to the lichen Collema limosum. 
Thenceforward their double resemblance, on the one hand to algae, on the 
other to fungi, was acknowledged, and influenced strongly the trend of study 
and investigation. 

The announcement 7 by Schwendener 8 of the dual hypothesis solved the 
problem for most students, though the relation between the two symbionts 
is still a subject of controversy. The explanation given by Schwendener, 
and still held by some 9 , that lichens were merely fungi parasitic on algae, 
was indeed a very inadequate conception of the lichen plant, and it was hotly 
contested by various lichenologists. Lauder Lindsay 10 dismissed the theory 
as " merely the most recent instance of German transcendentalism applied 

1 Luyken 1809. 2 Hornschuch 1819. 3 Raab 1819. 4 Dillenius 1741, p. 200. 

8 Wallroth 1825. 6 Agardh 1820. 7 See p. 27. 8 Schwendener 1867. 

9 Fink 1913. 10 Lindsay 1876. 



INTRODUCTION xxv 

to the Lichens." Earlier still, Nylander 1 , in. a paper dealing with cephalodia 
and their peculiar gonidia, had denounced it : " Locum sic suum dignum 
occupat algolichenomachia inter historias ridiculas, quae hodie haud paucae 
circa lichenes, majore imaginatione quam scientia, enarrantur." He never 
changed his attitude and Crombie 2 , wholly agreeing with his estimate of 
these " absurd tales," translates a much later pronouncement by him 3 : 
"All these allegations belong to inept Schwendenerism and scarcely deserve 
even to be reviewed or castigated so puerile are they the offspring of in- 
experience and of a light imagination. No true science there." Crombie 4 
himself in a first paper on this subject declared that " the new theory would 
necessitate their degradation from the position they have so long held as an 
independent class." He scornfully rejected the whole subject as "a Romance 
of Lichenology, or the unnatural union between a captive Algal damsel and 
a tyrant Fungal master." The nearest approach to any concession on the 
algal question occurs in a translation by Crombie 5 of one of Nylander's 
papers. It is stated there that a saxicolous alga (Gongrosira Kiitz.) had 
been found bearing the apothecia of Lecidea herbidula n. sp. Nylander adds : 
"This algological genus is one which readily passes into lichens." At a later 
date, Crombie 6 was even more comprehensively contemptuous and wrote: 
" whether viewed anatomically or biologically, analytically or synthetically, 
it is instead of being true science, only the Romance of Lichenology." These 
views were shared by many continental lichenologists and were indeed, as 
already stated, justified to a considerable extent: it was impossible to regard 
such a large and distinctive class of plants as merely fungi parasitic on the 
lower algae. 

Controversy about lichens never dies down, and that view of their para- 
sitic nature has been freshly promulgated among others by the American 
lichenologist Bruce Fink 7 . The genetic origin of the gonidia has also been 
restated by Elfving 8 : the various theories and views are discussed fully in 
the chapter on the lichen plant. 

Much of the interest in lichens has centred round their symbiotic growth. 
No theory of simple parasitism can explain the association of the two 
plants: if one of the symbionts is withdrawn either fungus or alga the 
lichen as such ceases to exist. Together they form a healthy unit capable 
of development and change : a basis for progress along new lines. Permanent 
characters have been formed which are transmitted just as in other units of 
organic life. 

A new view of the association has been advanced by F. and Mme Moreau 9 . 
They hold that the most characteristic lichen structures more particularly 

1 Nylander 1869. 2 Crombie 1891. 3 Nylander 1891. 4 Crombie 1874. 

8 Crombie 1877. 6 Crombie 1885. 7 Fink 1913. 8 Elfving 1913. 

9 Moreau 1918. 



xxvi INTRODUCTION 

the cortex have been induced by the action of the alga on the fungus. 
The larger part of the thallus might therefore be regarded as equivalent to 
a gall: "it is a cecidium, an algal cecidium, a generalized biomorphogenesis." 
The morphological characters of lichens are of exceptional interest, con- 
ditioned as they are by the interaction of the two symbionts, and new 
structures have been evolved by the fungus which provides the general 
tissue system. Lichens are plants of physiological symbiotic origin, and that 
aspect of their life-history has been steadily kept in view in this work. There 
are many new requirements which have had to be met by the lichen hyphae, 
and the differences between them and the true fungal hyphae have been 
considered, as these are manifested in the internal economy of the com- 
pound plant, and in its reaction to external influences such as light, heat, 
moisture, etc. 

The pioneers of botanical science were of necessity occupied almost 
exclusively with collecting and describing plants. As the number of known 
lichens gradually accumulated, affinities were recognized and more or less 
successful efforts were made to tabulate them in classes, orders, etc. It was 
a marvellous power of observation that enabled the early workers to arrange 
the first schemes of classification. Increasing knowledge aided by improved 
microscopes has necessitated changes, but the old fundamental "genus" 
Lichen is practically equivalent to the Class Lichenes. 

The study of lichens has been a slow and gradual process, with a con- 
tinual conflict of opinion as to the meaning of these puzzling plants their 
structure, reproduction, manner of subsistence and classification as well as 
their relation to other plants.. It has been found desirable to treat these 
different subjects from a historical aspect, as only thus can a true under- 
standing be gained, or a true judgment formed as to the present condition 
of the science. It is the story of the evolution of lichenology as well as of 
lichens that has yielded so much of interest and importance. 

The lichenologist may claim several advantages in. the study of his 
subject : the abundant material almost everywhere to hand in country 
districts, the ease with which the plants are preserved, and, not least, the 
interest excited by the changes and variations induced by growth conditions ; 
there are a whole series of problems and puzzles barely touched on as yet 
that are waiting to be solved. 

In field work, it is important to note accurately and carefully the nature 
of the substratum as well as the locality. Crustaceous species should be 
gathered if possible along with part of the wood or rock to which they are 
attached ; if they are scraped off, the pieces may be reassembled on gummed 
paper, but that is less satisfactory. The larger forms are more easily secured; 



INTRODUCTION xxvii 

they should be damped and then pressed before being laid away : the process 
flattens them, but it saves them from the risk of being crushed and broken, 
as when dry they are somewhat brittle. Moistening with water will largely 
restore their original form. All parts of the lichen, both thallus and fruit, 
can be examined with ease at any time as they do not sensibly alter in the 
herbarium, though they lose to some extent their colouring : the blue-grey 
forms, for instance, often become a uniform dingy brownish-grey. 

Microscopic examination in the determination of species is necessary in 
many instances, but that disability if it ranks as such is shared by other 
cryptogams, and may possibly be considered an inducement rather than a 
deterrent to the study of lichens. For temporary examination of microscopic 
preparations, the normal condition is best observed by mounting them in 
water. If the plants are old and dry, the addition of a drop or two of potash 
or ammonia solution is often helpful in clearing the membranes of the 
cells and in restoring the shrivelled spores and paraphyses to their natural 
forms and dimensions. 

If serial microtome sections are desired, more elaborate methods are 
required. For this purpose Peirce 1 has recommended that " when dealing 
with plants that are dry but still alive, the material should be thoroughly 
wetted and kept moist for two days, then killed and fixed in a saturated 
solution of corrosive sublimate in thirty-five per cent, alcohol." The solu- 
tion should be used hot : the usual methods of dehydrating and embedding 
in paraffin are then employed with extra precautions on account of the 
extremely brittle nature of lichens. 

Another method that also gave good results has been proposed by 
French 2 : " first the lichen is put into 95 per cent, alcohol for 24 hours, then 
into thin celloidin and thick celloidin 2/\. hours each. After this the specimens 
are embedded in thick celloidin which is hardened in 70 per cent, alcohol 
for 24. hours and then cut." French advises staining with borax carmine : 
it colours the fungal part pale carmine and the algal cells a greenish-red 
shade. 

Modern research methods of work are generally described in full in the 
publications that are discussed in the following chapters. The student is 
referred to these original papers for information as to fixing, embedding, 
staining, etc. 

Great use has been made of reagents in determining lichen species. 

They are extremely helpful and often give the clinching decision when 

morphological characters are obscure, especially if the plant has been much 

altered by the environment. It must be borne in mind, however, that a 

1 Peirce 1898. * French 1898. 



xxviii INTRODUCTION 

species is a morphological rather than a physiological unit, and it is not the 
structures but the cell-products that are affected by reagents. Those most 
commonly in use are saturated solutions of potash and of bleaching-powder 
(calcium hypochlorite). The former is cited in text-books as KOH or simply 
as K, the latter as CaCl or C. The C solution deteriorates quickly and 
must, therefore, be frequently renewed to produce the required reaction, 
i.e. some change of colour. These two reagents are used singly or, if con- 
jointly, K followed by C. The significance of the colour changes has been 
considered in the discussion on lichen-acids. 

Iodine is generally cited in connection with its staining effect on the 
hymenium of the fruit; the blue colour produced is, however, more general 
than was at one time supposed and is not peculiar to lichens ; the asci of 
many fungi react similarly though to a less extent. The medullary hyphae 
in certain species also stain blue with iodine. 



CHAPTER I 

HISTORY OF LICHENOLOGY 
A. INTRODUCTORY 

THE term "lichen" is a word of Greek origin used by Theophrastus in his 
History of Plants to signify a superficial growth on the bark of olive-trees. 
The name was given in the early days of botanical study not to lichens, as 
we understand them, but to hepatics of the Marchantia type. Lichens 
themselves were generally described along with various other somewhat 
similar plants as "Muscus" (Moss) by the older writers, and more definitely 
as "Musco-fungus" by Morison 1 . In a botanical work published in 170x3 by 
Tournefort'- all the members of the vegetable kingdom then known were 
for the first time classified in genera, and the genus Lichen was reserved for 
the plants that have been so designated since that time, though Dillenius 3 
in his works preferred the adjectival name Lichenoides. 

A painstaking historical account of lichens up to the beginning of 
modern lichenology has been written by Krempelhuber 4 , a German licheno- 
logist. He has grouped the data compiled by him into a series of Periods, 
each one marked by some great advance in knowledge of the subject, 
though, as we shall see, the advance from period to period has been con- 
tinuous and gradual. While following generally on the lines laid down by 
Krempelhuber, it will be possible to cite only the more prominent writers 
and it will be of much interest to British readers to note especially the work 
of our own botanists. 

Krempelhuber's periods are as follows: 

I. From the earliest times to the end of the seventeenth century. 
II. Dating from the arrangement of plants into classes called genera 
by Tournefort in 1694 to 1729. 

III. From Micheli's division of lichens into different orders in 1729 
to 1780. 

IV. The definite and reasoned establishment of lichen genera based 
on the structure of thallus and fruit by Weber in 1780 to 1803. 

V. The arrangement of all known lichens under their respective 

genera by Acharius in 1803 to 1846. 

VI. The recognition of spore characters in classification by De 
Notaris in 1846 to 1867. 

1 Morison 1699. 2 Tournefort 1694 and 1700. 3 Dillenius 1/41. 4 Krempelhuber 1867-1872. 
S. L. I 



2 HISTORY OF LICHENOLOGY 

A seventh period which includes modern lichenology, and which dates 
after the publication of Krempelhuber's History, was ushered in by 
Schwendener's announcement in 1867 of the hypothesis as to the dual 
nature of the lichen thallus. Schwendener's theory gave a new impulse to 
the study of lichens and strongly influenced all succeeding investigations. 

B. PERIOD I. PREVIOUS TO 1694 

Our examination of lichen literature takes us back to Theophrastus, 
the disciple of Plato and Aristotle, who lived from 371 to 2848.0., and who 
wrote a History of Plants, one of the earliest known treatises on Botany. 
Among the plants described by Theophrastus, there are evidently two 
lichens, one of which is either an Usnea or an Alectoria, and the other 
certainly Roccella tinctoria, the last-named an important economic plant 
likely to be well known for its valuable dyeing properties. The same or 
somewhat similar lichens are also probably alluded to by the Greek phy- 
sician Dioscorides, in his work on Materia Medica, A.D. 68. About the 
same time Pliny the elder, who was a soldier and traveller as well as a 
voluminous writer, mentions them in his Natural History which was 
completed in 77 A.D. 

During the centuries that followed, there was little study of Natural 
History, and, in any case, lichens were then and for a long time after 
considered to be of too little economic value to receive much attention. 

In the sixteenth century there was a great awakening of scientific 
interest all over Europe, and, after the printing-press had come into 
general use, a number of books bearing on Botany were published. It will 
be necessary to chronicle only those that made distinct contributions to the 
knowledge of lichens. 

The study of plants was at first entirely from a medical standpoint 
and one of the first works, and the first book on Natural History, printed 
in England, was the Crete Herball 1 . It was translated from a French work, 
Hortus sanitatis, and published by Peter Treveris in Southwark. One of 
the herbs recommended for various ailments is "Muscus arborum," the 
tree-moss (Usnea}. A somewhat crude figure accompanies the text. 

Ruel 2 of Soissons in France, Dorstenius 3 , Camerarius 4 and Tabernae- 
montanus 5 in Germany followed with works on medical or economic botany 
and they described, in addition to the tree-moss, several species of reputed 
value in the art of healing now known as Sticta (Lobaria) pulmonaria, 
Lobaria laetevirens, Cladonia pyxidata, Evernia prunastri and Cetraria 
islandica. Meanwhile L'Obel 6 , a Fleming, who spent the latter part of his 
life in England and is said to have had charge of a physic garden at 

1 Crete Herball 1526. Ruel I536 8 Dorstenius 1MO 

4 Camerarius 1586. 5 Tabernaemontanus 1590. 6 L'Obel 1576. 



PERIOD I. PREVIOUS TO 1694 3 

Hackney, was appointed botanist to James I. He published at Antwerp 
a large series of engravings of plants, and added a species of Ramalina to 
the growing list of recognized lichens. Dodoens 1 , also a Fleming, records 
not only the Usnea of trees, but a smaller and more slender black form 
which is easily identifiable as Alectoria jubata. He also figures Lichen 
pulmonaria and gives the recipe for its use. 

The best-known botanical book published at that time, however, is the 
Herball of John Gerard 2 of London, Master in Chirurgerie, who had a 
garden in Holborn. He recommends as medicinally valuable not only 
Usnea, but also Cladonia pyxidata, for which he coined the name "cuppe- 
or chalice-moss." About the same time Schwenckfeld 3 recorded, among 
plants discovered by him in Silesia, lichens now familiar as Alectoria 
jubata, Cladonia rangiferina and a species of Peltigera, 

Among the more important botanical writers of the seventeenth century 
may be cited Colonna 4 and Bauhin 5 . The former, an Italian, contributes, 
in his Ecphrasis, descriptions and figures of three additional species easily 
recognized as Physcia ciliaris, Xanthoria parietina and Ramalina calicaris. 
Kaspar Bauhin, a professor in Basle, who was one of the most advanced of 
the older botanists, was the first to use a binomial nomenclature for some 
of his plants. He gives a list in his Pinax of the lichens with which he was 
acquainted, one of them, Cladonia fimbriata, being a new plant. 

John Parkinson's 6 Herball is well known to English students; he adds 
one new species for England, Lobaria pulmonaria, already recorded on the 
Continent. Parkinson was an apothecary in London and held the office of 
the King's Herbarist; his garden was situated in Long Acre. How's 7 
Phytographia is notable as being the first account of British plants compiled 
without reference to their healing properties. Five of the plants described 
by him are lichen species: "Lichen arborum sive pulmonaria" (Lobaria 
pulmonaria}, "Lichen petraeus tinctorius" (Roccella}, "Muscus arboreus" 
(Usnea}, "Corallina montana" (Cladonia rangiferina} and "Muscus pixoides" 
(Cladonia}. Several other British species were added by Merrett 8 , who records 
in his Pinax, "Muscus arboreus umbilicatus" (Physcia dliaris}, "Muscus 
aureus tenuissimus" ( Teloschistes flavicans), "Muscus caule rigido" (Alec- 
toria) and "Lichen petraeus purpureus" (Parmelia omphalodes}, the last- 
named, a rock lichen, being used, he tells us, for dyeing in Lancashire. 

Merret or Merrett was librarian to the Royal College of Physicians. 
His Pinax was undertaken to replace How's Phytographia published 
sixteen years previously and then already out of print. Merrett's work 
was issued in 1666, but the first impression was destroyed in the great fire 
of London and most of the copies now extant are dated 1667. He arranged 

1 Dodoens 1583. 2 Gerard 1597. 3 Schwenckfeld 1600. 4 Colonna 1606. 

5 Bauhin 1623, pp. 360-2. * Parkinson 1640. 7 How 1650. 8 Merrett 1666. 



4 HISTORY OF LICHENOLOGY 

the species of plants in alphabetical order, but as the work was not critical 
it fell into disuse, being superseded by John Ray's Catalogus and Synopsis. 
To Robert Plot 1 we owe the earliest record of Cladonia cocci/era which had 
hitherto escaped notice; it was described and figured as a new and rare 
plant in the Natural History of Staffordshire^. Plot was the first Gustos 
of Ashmole's Museum in Oxford and he was also the first to prepare 
a County Natural History. 

The greatest advance during this first period was made by Robert 
Morison 2 , a Scotsman from Aberdeen. He studied medicine at Angers in 
France, superintended the Duke of Orleans' garden at Blois, and finally, 
after his return to this country in 1669, became Keeper of the botanic 
garden at Oxford. In the third volume of his great work 2 on Oxford 
plants, which was not issued till after his death, the lichens are put in 
a separate group "Musco-fungus" and classified with some other plants 
under "Plantae Heteroclitae." The publication of the volume projects into 
the next historical period. 

Long before this date John Ray had begun to study and publish books 
on Botany. His Catalogue of English Plants* is considered to have com- 
menced a new era in the study of the science. The Catalogue was followed 
by the History of Plants*, and later by a Synopsis of British Plants 5 , and in 
all of these books lichens find a place. Two editions of the Synopsis 
appeared during Ray's lifetime, and to the second there is added an 
Appendix contributed by Samuel Doody which is entirely devoted to 
Cryptogamic plants, including not a few lichens still called "Mosses" 
discovered for the first time. Doody, himself an apothecary, took charge 
of the garden of the Apothecaries' Society at Chelsea, but his chief interest 
was Cryptogamic Botany, a branch of the subject but little regarded before 
his day. Pulteney wrote of him as the "Dillenius of his time." 

Among Doody's associates were the Rev. Adam Buddie, James Petiver 
and William Sherard. Buddie was primarily a collector and his herbarium 
is incorporated in the Sloane Herbarium at the British Museum. It contains 
lichens from all parts of the world, many of them contributed by Doody, 
Sherard and Petiver. Only a few of them bear British localities : several are 
from Hampstead where Buddie had a church. 

The Society of Apothecaries had been founded in 1617 and the mem- 
bers acquired land on the river-front at Chelsea, which was extended later 
and made into a Physick Garden. James Petiver 6 was one of the first 
Demonstrators of Plants to the Society in connection with the 'garden, and 
one of his duties was to conduct the annual herborizing tours of the 
apprentices in search of plants. He thus collected a large herbarium on 
the annual excursions, as well as on shorter visits to the more immediate 

1 Plot 1686. * Morison 1699. 3 Ray 1670. 4 Ray 1686. 5 Ray 1690. 6 Petiver 1695. 



PERIOD I. PREVIOUS TO 1694 5 

neighbourhood of London. He wrote many tracts on Natural History 
subjects, and in these some lichens are included. He was one of the best 
known of Ray's correspondents, and owing to his connection with the 
Physic Garden received plants from naturalists in foreign countries. 

Sherard, another of Doody's friends, had studied abroad under Tournefort 
and was full of enthusiasm for Natural Science. It was he who brought 
Dillenius to England and finally nominated him for the position of the first 
Sherardian Professor of Botany at Oxford. Another well-known contem- 
porary botanist was Leonard Plukenet 1 who had a botanical garden at Old 
Palace Yard, Westminster. He wrote several botanical works in which 
lichens are included. 

Morison is the only one of all the botanists of the time who recognized 
lichens as a group distinct from mosses, algae or liverworts, and even he 
had very vague ideas as to their development. Malpighi 2 had noted the 
presence of soredia on the thallus of some species, and , regarded them as 
seeds. Porta 3 , a Neapolitan, has been quoted by Krempelhuber as probably 
the first to discover and place on record the direct growth of lichen fronds 
from green matter on the trunks of trees. 

C. PERIOD II. 1694-1729 

The second Period is ushered in with the publication of a French work, 
Les Elemens de Botatiique by Tournefort 4 , who was one of the greatest 
botanists of the time. His object was "to facilitate the knowledge of plants 
and to disentangle a science which had been neglected because it was found 
to be full of confusion and obscurity." Up to this date all plants were 
classified or listed as individual species. It was Tournefort who first 
arranged them in groups which he designated "genera" and he gave a 
careful diagnosis of each genus. 

Les Elemens was successful enough to warrant the publication a few 
years later of a larger Latin edition entitled Institutiones 5 and thus fitted for 
a wider circulation. Under the genus Lichen, he included plants "lacking 
flowers but with a true cup-shaped shallow fruit, with very minute pollen or 
seed which appeared to be subrotund under the microscope." Not only the 
description but the figures prove that he was dealing with ascospores and 
not merely soredia, though under Lichen along with true members of the 
"genus" he has placed a Marchantia, the moss Splachnum and a fern. A few 
lichens were placed by him in another genus Coralloides. 

Tournefort's system was of great service in promoting the study of 
Botany: his method of classification was at once adopted by the German 
writer Rupp 6 who published a Flora of plants from Jena. Among these 

1 Plukenet 1691-1696. 2 Malpighi 1686. 3 Porta 1688. 

4 Tournefort 1694. 5 Tournefort 1700. 6 Rupp 1718. 



6 HISTORY OF LICHENOLOGY 

plants are included twenty-five species of lichens, several of which he 
considered new discoveries, no fewer than five being some form of Lichen 
gelatinosus (Collema}. Buxbaum 1 , in his enumeration of plants from Halle, 
finds place for forty-nine lichen species, with, in addition, eleven species of 
Coralloides; and Vaillant 2 in listing the plants that grew in the neighbour- 
hood of Paris gives thirty-three species for the genus Lichen of which a 
large number are figured, among them species of Ramalina, Parmelia, 
Cladonia, etc. 

In England, however, Dillenius 3 , who at this time brought out a third 
edition of Ray's Synopsis and some years later his own Historia Muscorum, 
still described most of his lichens as "Lichenoides" or "Coralloides" ; and no 
other work of note was published in our country until after the Linnaean 
system of classification and of nomenclature was introduced. 

D. PERIOD III. 1729-1780 

Lichens were henceforth regarded as a distinct genus or section of 
plants. Micheli 4 , an Italian botanist, Keeper of the Grand Duke's Gardens 
in Florence, realized the desirability of still further delimitation, and he 
broke up Tournefort's large comprehensive genera into numerical Orders. 
In the genus Lichen, he found occasion for 38 of these Orders, determined 
mainly by the character of the thallus, and the position on it of apothecia 
and soredia. He enumerates the species, many of them new discoveries, 
though not all of them recognizable now. His great work on Plants is 
enriched by a series of beautiful figures. It was published in 1729 and 
marks the beginning of a new Period a new outlook on botanical science. 
Micheli regarded the apothecia of lichens as "floral receptacles," and the 
soredia as the seed, because he had himself followed the development of 
lichen fronds from soredia. 

The next writer of distinction is the afore-mentioned Dillen or 
Dillenius. He was a native of Darmstadt and began his scientific career 
in the University of Giessen. His first published work 5 was an account of 
plants that were to be found near Giessen in the different months of the 
year. Mosses and lichens he has assigned to December and January. 
Sherard induced him to come to England in 1721, and at first engaged his 
services in arranging the large collections of plants which he, Sherard, had 
brought from Smyrna or acquired from other sources. 

Three years after his arrival Dillenius had prepared the third edition of 
Ray's Synopsis for the press, but without putting his name on the title-page 6 . 
Sherard explained, in a letter to Dr Richardson of Bierly in Yorkshire, that 
"our people can't agree about an editor, they are unwilling a foreigner should 

1 Buxbaum 1721. 2 Vaillant 1727. Dillenius 1724 and 1741. 

4 Micheli 1729. 5 Dillenius 1719. s See Druce and yines IQ _ 



PERIOD III. 1729-1780 7 

put his name to it." Dillenius, who was quite aware of the prejudice against 
aliens, himself writes also to Dr Richardson : "there being some apprehension 
(me being a foreigner) of making natives uneasy if I should publicate it in 
my name." Lichens were already engaging his attention, and descriptions 
of 91 species were added to Ray's work. So well did this edition meet the 
requirements of the age, that the Synopsis remained the text-book of 
British Botany until the publication of Hudson's Flora Anglica in 1762. 

William Sherard died in 1728. He left his books and plates to the 
University of Oxford with a sum of money to endow a Professorship of 
Botany. In his will he had nominated Dr Dillenius for the post. The great 
German botanist was accordingly appointed and became the first Sherardian 
Professor of Botany, though he did not remove to Oxford till 1734. The 
following years were devoted by him to the preparation of Historia Mus- 
corum, which was finally published in 1741. It includes an account of the 
then known liverworts, mosses and lichens. The latter still considered by 
Dillenius as belonging to mosses were grouped under three genera, Usnea, 
Coralloides and Lichenoides. The descriptions and figures are excellent, and 
his notes on occasional lichen characteristics and on localities are full of 
interest. His lichen herbarium, which still exists at Oxford, mounted with 
the utmost care and neatness, has been critically examined by Nylander and 
Crombie 1 and many of the species identified. 

Dillenius was ignorant of, or rejected, Micheli's method of classification, 
adopting instead the form of the thallus as a guide to relationship. He also 
differed from him in his views as to propagation, regarding the soredia as 
the pollen of the lichen, and the apothecia as the seed-vessels, or even in 
certain .cases as young plants. 

Shortly after the publication of Dillenius' Historia, appeared Haller's 2 
Systematic and Descriptive list of plants indigenous to Switzerland. The 
lichens are described as without visible leaves or stamens but with "corpus- 
cula" instead of flowers and leaves. He arranged his lichen species, 160 in 
all, under seven different Orders: I. "Lichenes Corniculati and Pyxidati"; 
2. "L. Coralloidei"; 3. "L. Fruticosi"; 4. "L. Pulmonarii"; 5. "L. Crustacei" 
(with flower-shields); 6. "L. Scutellis" (with shields but with little or no 
thallus); and 7. "L. Crustacei" (without shields). 

This period extends till near the end of the eighteenth century, and 
thus includes within its scope the foundation of the binomial system of 
naming plants established by Linnaeus 3 . The renowned Swedish botanist 
rather scorned lichens as "rustici pauperrimi," happily translated by 
Schneider 4 as the "poor trash of vegetation," but he named and listed about 
80 species. He divided his solitary genus Lichen into sections: i. "Leprosi 
tuberculati"; 2. "Leprosi scutellati"; 3. "Imbricati"; 4. "Foliacei"; 

1 Crombie 1880. - Haller 1742. 3 Linnaeus 1753. 4 Schneider 1897. 



8 HISTORY OF LICHENOLOGY 

5. "Coriacei"; 6. "Scyphiferi"; 7. "Filamentosi." By this ordered sequence 
Linnaeus showed his appreciation of development, beginning, as he does, 
with the leprose crustaceous thallus and continuing up to the most highly 
organized filamentous forms. He and his followers still included the genus 
Lichen among Algae. 

A voluminous History of Plants had been published in 1751 by 
Sir John Hill 1 , the first superintendent to be appointed to the Royal 
Gardens, Kew. In the History lichens are included under the Class 
"Mosses," and are divided into several vaguely limited ''genera" Usriea, 
tree mosses, consisting of filaments only; Platysma, flat branched tree 
mosses, such as lungwort; Cladonia, the orchil and coralline mosses, such as 
Cladoniafurcata ; Pyxidium, the cup-mosses; and Placodium, the crustaceous, 
friable or gelatinous forms. A number of plants are somewhat obscurely 
described under each genus. Not only were these new Lichen genera sug- 
gested by him, but among his plants are such binomials as Usnea compressa, 
Platysmacorniculatum, Cladoniafurcata and Cladonia tophacea ; other lichens 
are trinomial or are indicated, in the way then customary, by a whole sen- 
tence. Hill's studies embraced a wide variety of subjects; he had flashes of 
insight, but not enough concentration to make an effective application of 
his ideas. In his Flora Britannica*, which was compiled after the publication 
of Linnaeus's Species Plantarum, he abandoned his own arrangement in 
favour of the one introduced by Linnaeus and accepted again the single 
genus Lichen. 

Sir William Watson 3 , a London apothecary and physician of scientific 
repute at this period, proposed a rearrangement and some alteration of 
Linnaeus's sections. He had failed to grasp the principle of development, 
but he gives a good general account of the various groups. Watson was the 
progenitor of those who decry the makers and multipliers of species. So in 
regard to Micheli, who had increased the number to "298," he writes: "it is to 
be regretted, that so indefatigable an author, one whose genius particularly 
led him to scrutinize the minuter subjects of the science, should have been 
so solicitous to increase the number of species under all his genera: an error 
this, which tends to great confusion and embarassment and must retard the 
progress and real improvement of the botanic science." Linnaeus however 
in redressing the balance earned his full approbation: "He has so far 
retrenched the genus (Lichen} that in his general enumeration of plants he 
recounts only 80 species belonging to it." 

Linnaeus's binomial system was almost at once adopted by the whole 
botanical world and the discovery and tabulation of lichens as well as of 
other plants proceeded apace. Scopoli's 4 Flora Carniolica, for instance, 
published in 1760, still adhered to the old descriptive method of nomen- 

1 Hill i7 5 i. Hill'sgenus Collema is Nostoc, etc. 2 Hill 1760. 8 Watson 1759. 4 Scopoli 1760. 



PERIOD III. 1729-1780 t 9 

clature, but a second edition, issued twelve years later, is based on the new 
system : it includes 54 lichen species. 

About this time Adanson 1 proposed a new classification of plants, 
dividing them into families, and these again into sections and genera. He 
transferred the lichens to the Family "Fungi," and one of his sections 
contains a number of lichen genera, the names of these being culled from 
previous workers, Dillenius, Hill, etc. A few new ones are added by himself, 
and one of them, Graphis, still ranks as a good genus. 

In England, Hudson 2 , who was an apothecary and became sub-librarian 
of the British Museum, followed Linnaeus both in the first and later editions 
of the Flora Anglica. He records 102 lichen species. Withering 3 w r as also 
engaged, about this time, in compiling his Arrangement of Plants. He 
translated Linnaeus's term "Algae" into the English word "Thongs," the 
lichens being designated as "Cupthongs." In later editions, he simply 
classifies lichens as such. Lightfoot 4 , whose descriptive and economic notes 
are full of interest, records 103 lichens in the Flora Scotica, and Dickson 5 
shortly after published a number of species from Scotland, some of them 
hitherto undescribed. Dickson was a nurseryman who settled in London, 
and his avocations kept him in touch with plant-lovers and with travellers 
in many lands. 

E. PERIOD IV. 1780-1803 

The inevitable next advance was made by Weber 6 who at the time was 
a Professor at Kiel. In a first work dealing with lichens he had followed 
Linnaeus; then he published a new method of classification in which the 
lichens are considered as an independent Order of Cryptogamia, and that 
Order, called "Aspidoferae," he subdivided into genera. His ideas had been 
partly anticipated by Hill and by Adanson, but the work of Weber indicates 
a more correct view of the nature of lichens. He established eight fairly 
well-marked genera, viz. Verrucaria, Tubercularia, Sphaerocephalum and 
Placodium,vf\\ic\\ were based on fruit-characters, the thallus being crustaceous 
and rather insignificant, and a second group Lichen, Collema, Cladonia and 
Usnea, in which the thallus ranked first in importance. Though Weber's 
scheme was published in 1780, it did not at first secure much attention. 
The great authority of Linnaeus dominated so strongly the botany of the 
period that for a long time no change was welcomed or even tolerated. 

In our own country Relhan at Cambridge and Sibthorp 7 at Oxford 
were making extensive studies of plants. The latter was content to follow 
Linnaeus in -his treatment of lichens. Relhan 8 also grouped his lichens 
under one genus though, in a second edition of his Flora, he broke away 
from the Linnaean tradition and adopted the classification of Acharius. 

1 Adanson 1763. 2 Hudson 1762 and 1778. 3 Withering 1776. * Lightfoot 1777. 

5 Dickson 1785. 6 Weber 1780. 7 Sibthorp 1794. 8 Relhan 1785 and 1820. 



I0 HISTORY OF LICHENOLOGY 

Extensive contributions to the knowledge of English plants generally 
were made by Sir James Edward Smith 1 who, in 1788, founded the Linnean 
Society of London of which he was President until his death in 1828. He 
began his great work, English Botany, in 1790 with James Sowerby as 
artist. Smith's and Sowerby 's part of the work came to an end in 1814; 
but a supplement was begun in 1831 by Hooker who had the assistance of 
Sowerby's sons in preparing the drawings. Nearly all the lichens recorded 
by Smith are published simply as Lichen, and his Botany thus belongs to 
the period under discussion, though in time it stretches far beyond. 

Continental lichenologists had been more receptive to new ideas, and 
other genera were gradually added to Weber's list, notably by Hoffmann 2 
and Persoon 3 . 

For a long time little was known of the lichens of other than European 
countries. Buxbaum 4 in the East, Petiver 5 and Hans Sloarie 6 in the West 
made the first exotic records. The latter notes how frequently lichens grew 
on the imported Jesuit's bark, and he quaintly suggests in regard to some 
of these species that they may be identical with the "hyssop that springeth 
out of the wall." It was not however till towards the end of the eighteenth 
century that much attention was given to foreign lichens, when Swartz 7 in 
the West Indies and Desfontaines 8 in N. Africa collected and recorded 
a fair number. Swartz describes about twenty species collected on his 
journey through the West Indian Islands (1783-87). 

Interest was also growing in other aspects of lichenology. Georgi 9 , a 
Russian Professor, was the first to make a chemical analysis of lichens. He 
experimented on some of the larger forms and extracted and examined the 
mucilaginous contents of Ramalina farinacea, Platystna glaucum, Lobaria 
pulmonaria, etc., which he collected from birch and pine trees. About this 
time also the French scientists Willomet 10 , Amoreux and Hoffmann jointly 
published theses setting forth the economic value of such lichens as were 
used in the arts, as food, or as medicine. 

F. PERIOD V. 1803-1846 

The fine constructive work of Acharius appropriately begins a new era 
in the history of lichenology. Previous writers had indeed included lichens 
in their survey of plants, but always as a somewhat side issue. Acharius 
made them a subject of special study, and by his scientific system of classifi- 
cation raised them to the rank of the other great classes of plants. 

Acharius was a country doctor at Wadstena on Lake Malar in Sweden, 
as he himself calls it, " the country of lichens." He was attracted to the 

1 Smith 1790. 3 Hoffmann 1798. 8 Persoon 1794. 4 Buxbaum 1728. 

5 Petiver 1712. 6 Sloane 1796 and 1807. 7 Swartz 1788 and 1791. s Desfontaines 1798-1800. 

9 Georgi 1797. 10 Willomet, etc. 1787. 



PERIOD V. 1803-1846 ii 

study of them by their singular mode of growth and organization, both of 
thallus and reproductive organs, for which reason he finally judged that 
lichens should be considered as a distinct Order of Cryptogamia. 

In his first tentative work 1 he had followed his great compatriot 
Linnaeus, classifying all the species known to him under the one genus 
Lichen, though he had progressed so far as to divide the unwieldy Genus 
into Families and these again into Tribes, these latter having each a tribal 
designation such as Verrucaria, Opegrapha, etc. He established in all twenty- 
eight tribes which, at a later stage, he transformed into genera after the 
example of Weber. 

Acharius, from the beginning of his work, had allowed great importance 
to the structure of the apothecia as a diagnostic character though scarcely 
recognizing them as true fruits. He gave expression to his more mature 
views first in the Methodus Lichenum*, then subsequently in the larger 
Lichenographia Universalia*. In the latter work there are forty-one genera 
arranged under different divisions; the species are given short and succinct 
descriptions, with habitat, locality and synonymy. No material alteration 
was made in the Synopsis Lichenum*, a more condensed work which he pub- 
lished a few years later. 

The Cryptogamia are divided by Acharius into six " Families," one of 
which, " Lichenes," is distinguished, he finds, by two methods of propagation : 
by propagula (soredia) and by spores produced in apothecia. He divides 
the family into classes characterized solely by fruit characters, and these 
again into orders, genera and species, of which diagnoses are given. With 
fuller knowledge many changes and rearrangements have been found 
necessary in the application and extension of the system, but that in no way 
detracts from the value of the work as a whole. 

' In addition to founding a scientific classification, Acharius invented 
a^lerminology for the structures peculiar to lichens. We owe to him the 
names and descriptions of " thallus," " podetium," " apothecium," " peri- 
thecium," "soredium," "cyphella" and "cephalodium," the last word how- 
ever with a different meaning from the one now given to it. He proposed 
several others, some of which are redundant or have fallen into disuse, but 
many of his terms as we see have stood thotest of time and have been 
found of service in allied branches of botany. J\ 

Lichens were studied with great zest by the men of that day. Hue 5 
recalls a rather startling incident in this connection: Wahlberg, it is said, 
had informed Dufour that he had sent a large collection of lichens from 
Spain to Acharius who was so excited on receiving them, that he fell ill 
and died in a few days (Aug. Hth, 1819). Dufour, however, had added the 
comment that the illness and death might after all be merely a coincidence. 

1 Acharius 1798. 2 Acharius 1803. 3 Acharius 1810. 4 Acharius 1814. Hue 1908. 



12 HISTORY OF LICHENOLOGY 

Among contemporary botanists, we find that De Candolle 1 in the volume 
he contributed to Lamarck's French Flora, quotes only from the earlier work 
of Acharius. He had probably not then seen the Methodus, as he uses none 
of the new terms ; the lichens of the volume are arranged under genera 
which are based more or less on the position of the apothecia on the thallus. 
Florke 2 , the next writer of consequence, frankly accepts the terminology 
and the new view of classification, though differing on some minor points. 

Two lists of lichens, neither of particular note, were published at this 
time in our country: one by Hugh Davies 3 for Wales, which adheres to the 
Linnaean system, and the other by Forster 4 of lichens round Tonbridge. 
Though Forster adopts the genera of Acharius, he includes lichens among 
algae. A more important publication was S. F. Gray's 5 Natural Arrange- 
ment of British Plants. Gray, who was a druggist in Walsall and afterwards 
a lecturer on botany in London, was only nominally 6 the author, as it was 
mainly the work of his son John Edward Gray 7 , sometime Keeper of Zoology 
in the British Museum. Gray was the first to apply the principles of the 
Natural System of classification to British plants, but the work was opposed 
by British botanists of his day. The years following the French Revolution 
and the Napoleonic wars were full of bitter feeling and of prejudice, and 
anything emanating, as did the Natural System, from France was rejected 
as unworthy of consideration. 

In the Natural Arrangement, Gray followed Acharius in his treatment 
of lichens ; but whereas Acharius, though here and there confusing fungus 
species with lichens, had been clear-sighted enough to avoid all intermixture 
of fungus genera, with the exception of one only, the sterile genus Rhizo- 
morpha, Gray had allowed the interpolation of several, such as Hysterium, 
Xylaria, Hypoxylon, etc. He had also raised many of Acharius's subgenera 
and divisions to the rank of genera, thus largely increasing their number. 
This oversplitting of well-defined genera has somewhat weakened Gray's 
work and he has not received from later writers the attention he deserves. 

The lichens of Hooker's 8 Flora Scotica, which is synchronous with Gray's 
work, number 195 species, an increase of about 90 for Scotland since the 
publication of Lightfoot's Flora more than 40 years before. Hooker also 
followed Acharius in his classification of lichens both in the Flora Scotica 
and in the Supplement to English Botany*, which was undertaken by the 
younger Sowerbys and himself. To that work Borrer (1781-1862), a keen 
lichenologist, supplied many new and rare lichens collected mostly in Sussex. 

It is a matter of regret that Greville should have so entirely ignored 
lichens in his great work on Scottish Cryptogams. The two species of 

1 De Candolle 1805. 2 Florke 18x5-1819. Davies 1813. * Forster :8i6. 5 S. F. Gray 1821. 
6 Carrington 1870. 7 See List of the Books, etc. by John Edward Gray, p. 3 1872 
8 Hooker 1821. Hooker 1831. " Greville 1823-1827. 



PERIOD V. 1803-1846 13 

Lichina are the only ones he figured, and these he took to be algae. He 1 was 
well acquainted with lichens, for in the Flora Edinensis he lists 128 species 
for the Edinburgh district, arranging the genera under "Lichenes" with the 
exception of Opegrapha and Verrucaria which are placed with the fungus 
genus Poronia in " Hypoxyla." Though he cites the publications of Acharius, 
he does not employ his scientific terms, possibly because he was writing his 
diagnoses in English. Two other British works of this time still remain to 
be chronicled : Hooker's 2 contributions to Smith's English Flora and 
Taylor's 3 work on lichens in Mackay's Flora Hibernica. Through these the 
knowledge of the subject was very largely extended in our country. 

The classification of lichens and their place in the vegetable kingdom 
were now firmly established on the lines laid down by Acharius. Fries 4 in 
his important work Lichenographia Europaea more or less followed his dis- 
tinguished countryman. The uncertainty as to the position and relationship 
of lichens had rendered the task of systematic arrangement one of peculiar 
difficulty and had unduly absorbed attention ; but now that a satisfactory 
order had been established in the chaos of forms, the way was clear for other 
aspects of the study. Several writers expressed their views by suggesting 
somewhat different methods of classification, others wrote monographs of 
separate groups, or genera. Fee 5 published an Essay on the Cryptogams 
(mostly lichens) that grew on officinal exotic barks; Florke 8 took up the 
difficult genus Cladonia\ Wallroth 7 also wrote on Cladonia\ Delise 8 on Sticta, 
and Chevalier 9 published a long and elaborate account of Graphideae. 

Wallroth and Meyer at this time published, simultaneously, important 
studies on the general morphology and physiology of lichens. Wallroth 10 
had contemplated an even larger work on the Natural History of Lichens, 
but only two of the volumes reached publication. In the first of these he 
devoted much attention to the " gonidia " or " brood-cells " and established 
the distinction between the heteromerous and homoiomerous distribution of 
green cells within the thallus; he also describes with great detail the "mor- 
phosis" and "metamorphosis" of the vegetative body. In the second volume 
he discusses their physiology the contents and products of the thallus, 
colouring, nutrition, season of development, etc. and finally the pathology 
of these organisms. He made no great use of the compound microscope, 
and his studies were confined to phenomena that could be observed with a 
single lens. 

Meyer's 11 work contains a still more exact study of the anatomy and 
physiology of lichens; he also devotes many passages to an account of their 
metamorphoses, pointing out that species alter so much in varying conditions, 
that the same one at different stages may be placed even in different genera; 

1 Greville 1824. - Hooker 1833. 3 Taylor 1836. 4 Fries 1831. 5 Fee 1874. 6 Florke 1828. 

7 Wallroth 1829. 8 Delise 1822. Chevalier 1824. 10 Wallroth 1825. " Meyer 1825. 



I4 HISTORY OF LICHENOLOGY 

he however carries his theory of metamorphosis too far and unites together 
widely separated plants. Meyer was the first to describe the growth of the 
lichen from spores, though his description is somewhat confused. Possibly 
the honour of havingfirst observed their germination should be given to a later 
botanist, Holle 1 . The works of both Wallroth and Meyer enjoyed a great 
and well-merited reputation : they were standard books of consultation for 
many years. Koerber 2 , who devoted a long treatise to the study of gonidia, 
confirmed Wallroth's theories: he considered at that time that the gonidia 
in the soredial condition were organs of propagation. 

Mention should be made here of the many able and keen collectors who, 
in the latter half of the eighteenth century and the beginning of the nine- 
teenth, did so much to further the knowledge of lichens in the British Isles. 
Among the earliest of these naturalists are Richard Pulteney (1730-1801), 
whose collection of plants, now in the herbarium of the British Museum, in- 
cludes many lichens, and Hugh Davies (1739-1821), a clergyman whose 
Welsh plants also form part of the Museum collection. The Rev. John 
Harriman (1760-1831) sent many rare plants from Egglestone in Durham 
to the editors of English Botany and among them were not a few lichens. 
Edward Forster (1765-1849) lived in Essex and collected in that county, 
more especially in and near Epping Forest, and another East country 
botanist, Dawson Turner (17/5-1858), though chiefly known as an algologist, 
gave considerable attention to lichens. In Scotland the two most active 
workers were Charles Lyell (1767-1849), of Kinnordy in Forfarshire, and 
George Don (1798-1 856), also a Forfar man. Don was a gardener and became 
eventually a foreman at the Chelsea Physic Garden. Sir Thomas Gage of 
Hengrave Hall (1781-1823) botanized chiefly in his own county of Suffolk ; 
but most of his lichens were collected in South Ireland and are incorporated in 
the herbarium of the British Museum. Miss Hutchins also collected in Ireland 
and sent her plants for inclusion in English Botany. But in later years, the 
principal lichenologist connected with that great undertaking was W. Borrer, 
who spent his life in Sussex : he not only supplied a large number of specimens 
to the authors, but he himself discovered and described many new lichens. 

American lichenologists were also extremely active all through this 
period. The comparatively few lichens of Michaux's 3 Flora grouped under 
" Lichenaceae " were collected in such widely separated regions as Carolina 
and Canada. A few years later Miihlenberg 4 included no fewer than 184 
species in his Catalogue of North American Plants. Torrey 6 and Halsey 6 
botanized over a limited area near New York, and the latter, who devoted 
himself more especially to lichens, succeeded in recording 176 different forms, 
old and new. These two botanists were both indebted for help in their work 

1 Holle 1849. 2 Koerber 1839. 3 Michaux 1803. 

4 Muhlenberg 1813. 5 Torrey 1819. * Halsey 1824. 



PERIOD V. 1803-1846 15 

to Schweinitz, a Moravian brother, who moved from one country to another, 
working and publishing, now in America and now in Europe. His name is 
however chiefly associated with fungi. Later American lichenology is 
nobly represented by Tuckerman 1 who issued his first work on lichens in 
1839, and who continued for many years to devote himself to the subject. 
He followed at first the classification and nomenclature that had been 
adopted by Fee, but as time went on he associated himself with all that was 
best and most enlightened in the growing science. 

Travellers and explorers in those days of high adventure were constantly 
sending their specimens to European botanists for examination and deter- 
mination, and the knowledge of exotic lichens as of other classes of plants 
grew with opportunity. Among the principal home workers in foreign 
material, at this time, may be cited Fee 2 who described a very large series 
on officinal barks {Cinchona, etc.) so largely coming into use as medicines; 
he also took charge of the lichens in Martius's 3 Flora of Brazil. Montagne 4 
named large collections, notably those of Leprieur collected in Guiana, and 
Hooker 5 and Walker Arnott determined the plants collected during Captain 
Beechey's voyage, which included lichens from many different regions. 

G. PERIOD VI. 1846-1867 

The last work of importance, in which microscopic characters were 
ignored, was the Enumeratio critica Lichenum Europaeum by Schaerer 6 , a 
veteran lichenologist, who rather sadly realized at the end the limitations 
of that work, as he asks the reader to accept it " such as it is." Many years 
previously, Eschweiler 7 in his Systema and Fee 8 in his account of Cryptogams 
on Officinal Bark, had given particular attention to the internal structure as 
well as to the outward form of the lichen fructification. Fe"e, more especially, 
had described and figured a large number of spores; but neither writer had 
done more than suggest their value as a guide in the determination of genera 
and species. 

It was an Italian botanist, Giuseppe de Notaris 9 , a Professor in Florence, 
who took up the work where Fee had left it. His comparative studies of both 
vegetative and reproductive organs convinced him of the great importance 
of spore characters in classification, the spore being, as he rightly decided, 
the highest and ultimate product of the lichen plant. In his microscopic 
examination of the various recognized genera, he found that while, in some 
genera, the spores conformed to one distinct type, in others their diversities 
in form, septation or colour gave a decisive reason for the establishment of 
new genera, while minor differences in size, etc. of the spores proved to be of 
great value in distinguishing species. The spore standard thus marks a new 

1 Tuckerman 1839. 2 Fee 1824. s Martius 1833. 4 Montagne 1851. 5 Hooker 1841. 
6 Schaerer 1850. ~ Eschweiler 1824. 8 Fee 1824. 9 De Notaris 1846. 



16 HISTORY OF LICHENOLOGY 

departure in lichenology. De Notaris published the results of his researches 
in a fragment of a projected larger work that was never completed. Though 
his views were overlooked for a time, they were at length fully recognized 
and further elaborated by Massalongo 1 in Italy, by Norman 2 in Norway, by 
Koerber 3 in Germany and by Mudd 4 in our own country. Massalongo had 
drawn up the scheme of a great Scolia Lichenographica, but like de Notaris, 
he was only able to publish a part. After twelve years of ill-health, in which 
he struggled to continue his work, he died at the early age of 36. 

Lindsay 5 , Mudd and Leighton 6 were at this time devoting great attention 
to British lichens. Lauder Lindsay's Popular History of British Lichens, 
with its coloured plates and its descriptive and economic account of these 
plants has enabled many to acquire a wide knowledge of the group. Mudd's 
Manual, a more complete and extremely valuable contribution to the subject, 
followed entirely on the lines of Massalongo's work. From his large 
experience in the examination of lichens he came to the conclusion that : 
" Of all organs furnished by a given group of plants, none offer so many 
real, constant and physiological characters as the spores of lichens, for the 
formation of a simple and natural classification." 

Meanwhile, a contemporary writer, William Nylander, was rising into 
fame. He was born at Uleaborg in Finland 7 in 1822 and became interested 
in lichens very early in his career. His first post was the professorship of 
botany at Helsingfors; but in 1863 he gave up his chair and removed to 
Paris where he remained, except for short absences, until his death. One 
of his excursions brought him to London in 1857 to examine Hooker's 
herbarium. He devoted his whole life to the study of lichens, and from 
1852, the date of his first lichen publication, which is an account of the lichens 
of Helsingfors, to the end of his life he poured out a constant succession of 
books or papers, most of them in Latin. One of his earliest works was an 
Essay on Classification 91 which he elaborated later, but which in its main 
features he never altered. He relied, in his system, on the structure and form 
of thallus, gonidia and fructifications, more especially on those of the 
spermogonia (pycnidia), but he rejected ascospore characters except so far as 
they were of use in the diagnosis of species. He failed by being too isolated 
and by his unwillingness to recognize results obtained by other workers. 
In 1866 he had discovered the staining reactions of potash and hypochlorite 
of lime on certain thalli, and though these are at times unreliable owing to 
growth conditions, etc., they have generally been of real service. Nylander, 
however, never admitted any criticism of his methods; his opinions once 
stated were never revised. He rejected absolutely the theory of the dual 
nature of lichens propounded by Schwendener without seriously examining 

1 Massalongo 1852. Norman 1852. - 3 Koerber 1855. 4 Mudd 1861. 

5 Lindsay 1856. Leighton 1851, etc. 7 See Hue 1899. 8 Nylander 1854 and 1855. 



PERIOD VI. 1846-1867 17 

the question, and regarded as personal enemies those who dared to differ 
from him. The last years of his life were passed in complete solitude. He 
died in March 1899. 

Owing to the very inadequate powers of magnification at the service of 
scientific workers, the study of lichens as of other plants was for long restricted 
to the collecting, examining and classifying of specimens according to their 
macroscopic characters; the microscopic details observed were isolated and 
unreliable except to some extent for spore characters. Special interest is 
therefore attached to the various schemes of classification, as each new one 
proposed reflects to a large extent the condition of scientific knowledge of 
the time, and generally marks an advance. It was the improvement of the 
microscope from a scientific toy to an instrument of research that opened 
up new fields of observation and gave a new impetus to the study of a group 
of plants that had proved a puzzle from the earliest times. 

Tulasne was one of the pioneers in microscopic botany. He made 
a methodical study of a large series of lichens 1 and traced their develop- 
ment, so far as he was able, from the spore onwards. He gave special attention 
to the form and function of spermogonia and spermatia, and his work is 
enriched by beautiful figures of microscopic detail. Lauder Lindsay 2 also 
published an elaborate treatise on spermogonia, on their occurrence in the 
lichen kingdom and on their form and structure. The paper embodies the 
results of wide microscopic research and is a mine of information regarding 
these bodies. 

Much interesting work was contributed at this time by Itzigsohn 3 , 
Speerschneider 4 , Sachs 5 , Thwaites 6 , and others. They devoted their researches 
to some particular aspect of lichen development and their several contribu- 
tions are discussed elsewhere in this work. 

Schwendener 7 followed with a systematic study of the minute anatomy 
of many of the larger lichen genera. His work is extremely important in 
itself and still more so as it gradually revealed to him the composite 
character of the thallus. 

Several important monographs date from this period : Leighton 8 reviewed 
all the British " Angiocarpous " lichens with special reference to their 
" sporidia " though without treating these as of generic value. He followed 
up this monograph by two others, on the Graphideae 9 and the Umbili- 
carieae, and Mudd 11 published a careful study of the British Cladoniae. 
On the Continent Th. Fries 12 issued a revision of Stereocaulon and Pilo- 
pkoron and other writers contributed work on smaller groups. 

1 Tulasne 1852. 2 Lauder Lindsay 1859. 3 Itzigsohn 1854-1855. 4 Speerschneider 1853. 

5 Sachs 1855. fi Thwaites 1849. 7 Schwendener 1863-1868. 8 Leighton 1851. 

9 Leighton 1854. 10 Leighton 1856. " Mudd 1865. 12 Th. Fries 1858. 



I8 HISTORY OF LICHENOLOGY 



H. PERIOD VII. 1867 AND AFTER 

Modern lichenology begins with the enunciation of Schwendener's 1 theory 
of the composite nature of the lichen plant. The puzzling resemblance of 
certain forms to algae, of others to fungi, had excited the interest of botanists 
from a very early date, and the similarity between the green cells in the 
thallus, and certain lower forms of algae had been again and again pointed 
out. Increasing observation concerning the life-histories of these algae and 
of the gonidia had eventually piled up so great a number of proofs of their 
identity that Schwendener's announcement must have seemed to many an 
inevitable conclusion, though no one before had hazarded the astounding 
statement that two organisms of independent origin were combined in the 
lichen. 

f The dual hypothesis, as it was termed, was not however universally 
accepted. It was indeed bitterly and scornfully rejected by some of the 
most prominent lichenologists of the time, including Nylander 2 , J. Miiller 
and Crombie 3 . Schwendener held that the lichen was a fungus parasitic 
on an alga, and his opponents judged, indeed quite rightly, that such a view 
was wholly inadequate to explain the biology of lichens. It was not till a 
later datgjhat the truer conception of the "consortium" or "symbiosis" was 
proposed. ^T he researches undertaken to prove or disprove the new theories 
cojne under review in Chapter II. 

( Stahl's work on the development of the carpogonium in lichens gave a 
rte^direction to study, and notable work has beeadone during the last forty 
years in that as in other branches of lichenology./ 

Exploration of old and new fields furnished the lichen-flora of the world 
with many new plants which have been described by various systematists 
by Nylander, Babington, Arnold, Mujler, Th. Fries, Stizenberger, Leighton, 
Crombie and many others, and their contributions arc scattered through 
contemporary scientific journals. The number of recorded species is now 
somewhere about 40,00x3, though, in all probability, many of these will be 
found to be growth forms. Still, at the lowest computation, the number of 
different species is very large. 

Systematic literature has been enriched by a series of important mono- 
graphs, too numerous to mention here. While treating definite groups, they 
have helped to elucidate some of the peculiar biological problems of the 
symbiotic growth. 

Morphology, since Schwendener's time, has been well represented by 
Zukal, Reinke, Lindau, Funfstiick, Darbishire, Hue, and by an increasing 
number of modern writers whose work is duly acknowledged under each 

1 Schwendener 1867. n - Nylander 1874. :i Croml.ie 1885. 



PERIOD VII. 1867 AND AFTER 19 

subject of study. Hesse and Zopf, and more recently Lettau, have been 
engaged in the examination of those unique products, the lichen acids, while 
other workers have investigated lichen derivatives such as fats. Ecology of 
lichens has also been receiving increased attention. Problems of physiology, 
symbiosis, etc., are not yet considered to be solved and are being attacked 
from various sides. 

British lichenologists since 1867 have been mainly engaged on field 
work, with the exception of Lauder Lindsay who published after that date 
a second great paper on the spermogonia of crustaceous lichens. Leighton 
in his Lichen Flora and Crombie in numerous publications gave the lead in 
systematic work, and with them were associated a band of indefatigable 
collectors. Among these may be recalled Alexander Croall (1809-85), a 
parish schoolmaster in Scotland whose Plants of Braemar include many of 
the rarer mountain lichens. Henry Buchanan Holl (1820-86), a surgeon in 
London, collected in the Scottish Highlands as well as in England and 
Wales. William Joshua (1828-98) worked mostly in the Western counties 
of Somerset and Gloucestershire. Charles Du Bois Larbalestier, who died in 
191 1, was a keen observer and collector during many years; he discovered 
a number of new species in his native Jersey, in Cambridgeshire and also in 
Connemara; his plants were generally sent to Nylander to be determined 
and described. He issued two sets of lichens, one of Channel Island plants, 
the other of more general British distribution, and he had begun the issue 
of Cambridgeshire lichens. Isaac Carroll (1828-80), an Irish botanist, issued 
a first fascicle of Lichenes Hibernici containing 40 numbers. More recently 
Lett 1 has reported 80 species and varieties from the Mourn e Mountains in 
Ireland. Other more extensive sets were issued by Mudd and by Leighton, 
and later by Crombie and by Johnson. All these have been of great service 
to the study of lichenology in our country. Other collectors of note are 
Curnow (Cornwall), Martindale (Westmoreland), and E. M. Holmes whose 
valuable herbarium has been secured by University College, Nottingham. 

The publication of the volume dealing with Lichenes in Engier and 
Prantl's Pflanzenfamilien has proved a boon to all who are interested in the 
study of lichens. Fiinfstuck 2 prepared the introduction, an admirable 
presentation of the morphological and physiological aspects of the subject, 
while Zahlbruckner 3 , with equal success, took charge of the section dealing 
with classification. 

1 Lett 1890. - Fiinfstiick 1898. :t Zahlbruckner 1903-1907. 



CHAPTER II 



CONSTITUENTS OF THE LICHEN THALLUS 
I. LICHEN GONIDIA 

THE thallus or vegetative body of lichens differs from that of other green 
plants in the sharp distinction both of form and colour between the assimi- 
lative cells and the colourless tissues, and in the relative positions these 
occupy within the thallus: in the greater number of lichen species the green 
chlorophyll cells are confined to a narrow zone or band some way beneath 
and parallel with the surface (Fig. i); in a minority of genera they are dis- 
tributed through the entire thallus (Fig. 2); but in all cases the tissues 





Fig. i. Physcia aipolia Nyl. Vertical 
section of thallus. a, cortex; b, algal 
layer; c, medulla; d, lower cortex, 
x 100 (partly diagrammatic). 



Fig. i. Collema ntgrescens Ach. Vertical 
section of thallus. , chains of the 
alga Nostoc ; b, fungal filaments, x 600. 

remain distinct. The green zone can be easily demonstrated in any of the 
larger lichens by scaling off the outer surface cells, or by making a vertical 
section through the thallus. The colourless cells penetrate to some extent 
among the green cells; they also form the whole of the cortical and 
medullary tissues. 

These two different elements we now know to consist of two distinct 
organisms, a fungus and an alga. The green algal cells were at one time 
considered to be reproductive bodies, and were called "gonidia," a term still 
in use though its significance has changed. 



LICHEN GONIDIA 21 

i. GONIDIA IN RELATION TO THE THALLUS 

A. HISTORICAL ACCOUNT OF LICHEN GONIDIA 
There have been few subjects of botanical investigation that have 
roused so much speculation and such prolonged controversy as the question 
of these constituents of the lichen plant. The green cells and the colourless 
filaments which together form the vegetative structure are so markedly 
dissimilar, that constant attempts have been made to explain the problem 
of their origin and function, and thereby to establish satisfactorily the 
relationship of lichens to other members of the Plant Kingdom. 

In gelatinous lichens, represented by Collema, of which several species 
are common in damp places and grow on trees or walls or on the ground, 
the chains of green cells interspersed through the thallus have long been 
recognized as comparable with the filaments of Nostoc, a blue-green 
gelatinous alga, conspicuous in wet weather in the same localities as those 
inhabited by Collema. So among early systematists, we find Ventenat 1 
classifying the few lichens with which he was acquainted under algae and 
hazarding the statement that a gelatinous lichen such as Collema was only 
a Nostoc changed in form. Some years later Cassini 2 in an account of Nostoc 
expressed a somewhat similar view, though with a difference: he suggested 
that Nostoc was but a monstrous form of Collema, his argument being that, 
as the latter bore the fruit, it was the normal and perfect condition of 
the plant. A few years later Agardh 3 claimed to have observed the meta- 
morphosis of Nostoc up to the fertile stage of a lichen, Collema limosum. 
But long before this date, Scopoli 4 had demonstrated a green colouring 
substance in non-gelatinous lichens by rubbing a crustaceous or leprose 
thallus between the fingers; and Persoon 5 made use of this green colour 
characteristic of lichen crusts to differentiate these plants from fungi. 
Sprengel 6 went a step further in exactly describing the green tissue as 
forming a definite layer below the upper cortex of foliaceous lichens. 

The first clear description and delimitation of the different elements 
composing the lichen thallus was, however, given by Wallroth 7 . He drew 
attention to the great similarity between the colourless filaments of the 
lichen and the hyphae of fungi. The green globose cells in the chlorophylla- 
ceous lichens he interpreted as brood-cells or gonidia, regarding them as 
organs of reproduction collected into a "stratum gonimon." To the same 
author we owe the terms "homoiomerous" and "heteromerous," which he 
coined to describe the arrangement of these green cells in the tissue of the 
thallus. In the former case the gonidia are distributed equally through the 
structure; in the latter they are confined to a distinct zone. 

1 Ventenat 1794, p. 36. 2 Cassini 1817, p. 395. 3 Agardh 1820. 4 Scopoli 1/60, p. 79. 
5 Persoon 1794, p. 17. 6 Sprengel 1804, p. 325. 7 Wallroth 1825, I. 



22 CONSTITUENTS OF THE LICHEN THALLUS 

Wallroth's terminology and his views of the. function of the gonidia were 
accepted as the true explanation for many years, the opinion that they were 
solely reproductive bodies being entirely in accordance with the well-known 
part played by soredia in the propagation of lichens and soredia always 
include one or more green cells. 

B. GONIDIA CONTRASTED WITH ALGAE 

In describing the gonidia of the Graphideae Wallroth 1 had pointed out 
their affinity with the filaments of Chroolepus ( Trentepohlid) umbrina. He 
considered these and other green algae when growing 10986 on the trunks of 
trees to be but "unfortunate brood-cells" which had become free and, though 
capable of growth and increase, were unable to form again a lichen plant. 

Further observations on gonidia were made by E. Fries 2 : he found that 
the green cells escaped from the lichen matrix and produced new individuals; 
and also that the whole thallus in moist localities might become dissolved 
into the alga known as Protococcus viridis\ but, he continues, "though these 
Protococcus cells multiplied exceedingly, they never could rise again to the 
perfect lichen." Kiitzing 3 , in a later account of Protococcus viridis, also 
recognized its affinity with lichens; he stated that he could testify from 
observation that, according to the amount of moisture present, it would 
develop, either in excessive moisture to a filamentous alga, or in drier con- 
ditions "to lichens such as Lecanora subfusca or Xanthoria parietina." 

A British botanist, G. H. K. Thwaites 4 , at one time -superintendent of 
the botanical garden at Peradeniya in Ceylon, published a notable paper 
on lichen gonidia in which he pointed out 
that as in Collema the green constituents 
of the thallus resembled the chains of Nostoc, 
so in the non-gelatinous lichens, the green 
globose cells were comparable or identical with 
Pleurococctis, and Thwaites further observed that 
they increased by division within the lichen 
thallus. He insisted too that in no instance were 
gonidia reproductive organs : they were essen- 
tial component parts of the vegetative body and 
necessary to the life of the plant. In a further 

paper on Chroolepus cbeneus Ag., a plant con- 
Fig. 3. Coenogomum ebeneum A. L. . . . . , 

Sm. Tip ofiichen filament, the alga sisting of slender dark-coloured felted fila- 

overgrown by dark fungal hyphae men ts, he described these filaments as being 

composed of a central strand which closely 

resembled the alga Chroolepus, and of a surrounding sheath of dark-coloured 

1 Wallroth 1825, 1, p. 303. * Fries 1831, pp. Ivi and Ivii. 

3 Kutzing 1843. 4 Thwaites 1849, pp. 219 and 241. 




LICHEN GONIDIA 



cells (Fig. 3): "occasionally," he writes, "the internal filament protrudes 
beyond the investing sheath, and may then be seen to consist of oblong 
cells containing the peculiar reddish oily-looking endochrome of Chroolepus? 
Thwaites placed this puzzling plant in a new genus, Cystocolens, at the same 
time pointing out its affinity with the lichen genus Coenogoninm. The 
plant is now known as Coenogonium ebeneum. Thwaites was on the 
threshold of the discovery as to the true nature of the relationship between 
the central filament and the investing sheath, but he failed to take the next 
forward step. 

Very shortly after, Von Flotow 1 published his views on some other 
lichen gonidia. He had come to the conclusion that the various species of 
the alga, Gloeocapsa, so frequently found in damp places, among mosses and 
lichens, were merely growth stages of the gonidia of Ephebe pubescens, and 
bore the same relation to Ephebe as did Lepra viridis (Protococcus) to Par- 
melia. The gonidium of Ephebe is the gelatinous 
filamentous blue-green alga Stigonema (Fig. 4), 
and the separate cells are not unlike those of 
Gloeocapsa. Flotow had also demonstrated that the 
same type of gonidium was enclosed in the cepha- 
lodia of Stereocaulon. Sachs 2 , tob, gave evidence as 
to the close connection between Nostoc and Col- 
lema. He had observed numerous small clumps of 
the alga growing in proximity to equally abundant 
thalli of Collema, with every stage of development 
represented from one to the other. He found cases 
where the gelatinous coils of Nostoc chains were 
penetrated by fine colourless filaments "as if in- 
vaded by a parasitic fungus." Later these threads were seen to be attached 
to some cell of the Nostoc trichome. Sachs concluded, however, from very 
careful examination at the time, that the colourless filaments were produced 
by the green cells. As growth proceeded, the coloured Nostoc chains became 
massed towards the upper surface, while the colourless filaments tended to 
occupy the lower part of the thallus. He calculated that during the summer 
season the metamorphosis from Nostoc to a fertile Collema thallus took from 
three to four months. He judged that in favourable conditions the change 
would inevitably take place, though if there should be too great moisture no 
Collema would be formed. His study of Cladonia was less successful as he 
mistook some colonies of Gloeocapsa for a growth condition of Cladonia 
gonidia, an error corrected later by Itzigsohn 3 . 

But before this date Itzigsohn 4 had published a paper setting forth his 
views on thallus formation, which marked a distinct advance. He did not 
1 Flotow 185^. - Sachs 1855. 3 Itzigsohn 1855. 4 Itzigsohn 1854. 




Fig. 4. Ephebe pubescens Nyl. 
Tip of lichen filament >: 600. 



24 CONSTITUENTS OF THE LICHEN THALLUS 

hazard any theory as to the origin of gonidia, but he had observed spermatia 
growing, much as did the cells of Oscillaria: by increase in length, and, by 
subsequent branching, filaments were formed which surrounded the green 
cells; these latter had meanwhile multiplied by repeated division till finally 
a complete thallus was built up, the filamentous tissue being derived from 
the spermatia, while the green layer came from the original gonidium. In 
contrasting the development with that of Collema, he represents Nostoc as 
a sterile product of a lichen and, like the gonidia of other lichens, only able 
to form a lichen thallus when it encounters the fructifying spermatia. 

Braxton Hicks 1 , a London doctor, some time later, made experiments 
with Chroococcus algae which grew in plenty on the bark of trees, and 
followed their development into a lichen thallus. He further claimed to 
have observed a C/ilorococcus, which was associated with a Cladonia, divide 
and form a Palmella stage. 

C. CULTURE EXPERIMENTS WITH THE LICHEN THALLUS 

It had been repeatedly stated that the gonidia might become independent 
of the thallus, but absolute proof was wanting until Speerschneider 2 , who 
had turned his attention to the subject, made direct culture experiments 
and was able to follow the liberation of the green cells. He took a thinnish 
section of the thallus of Hagenia (Pkyscia) ciliaris, and, by keeping it moist, 
he was able to observe that the gonidial cells increased by division; the 
moist condition at the same time caused the colourless filaments to die 
away. This method of investigation was to lead to further results. It was 
resorted to by Famintzin and Baranetsky 3 who made cultures of gonidia 
extracted from three different lichens, Physcia (Xanthorid) parietina, 
Evernia furfur acea and Cladonia sp. They were able to observe the growth 
and division of the green cells and, in addition, the formation of zoospores. 
They recognized the development as entirely identical with that of the 
unicellular green alga, Cystococcus humicola Naeg. Baranetsky 4 continued 
the experiments and made cultures of the blue-green gonidia of Peltigera 
canina and of Collema pulposum. In both instances he succeeded in isolating 
them from the thallus and in growing them in moist air as separate 
organisms. He adds that "many forms reckoned as algae, may be con- 
sidered as vegetating lichen gonidia such as Cystococcus, Polycoccus, Nostoc, 
etc." Meanwhile Itzigsohn 5 had further demonstrated by similar culture 
experiments that the gonidia of Peltigera canina corresponded with the 
algae known as Gloeocapsa monococca Kiitz., and as Polycoccus punctiformis 
Kiitz. 

1 Hicks 1860 and 1861. 2 Speerschneider 1853. = Famintzin and Baranetsky 1867. 
4 Baranetsky 1869. 5 Itzigsohn 1867. 



LICHEN GONIDIA 25 

D. .THEORIES AS TO THE ORIGIN OF GONIDIA 

Though the relationship between the gonidia within the thallus and free- 
living algal organisms seemed to be proved beyond dispute, the manner in 
which gonidia first originated had not yet been discovered. Bayrhoffer 1 
attacked this problem in a study of foliose and other lichens. According 
to his observations, certain colourless cells or filaments, belonging to the 
"gonimic" layer, grew in a downward direction and formed at their tips a 
faintly yellowish-green cell ; it gradually enlarged and was at length thrown 
off as a free globose gonidium, which represented the female cell. Other 
filaments from the "lower fibrous layer" of the thallus at the same time grew 
upwards and from them were given off somewhat similar gonidia which 
functioned as male cells. His observations and deductions, were fanciful, 
but it must be remembered that the attachment between hypha and alga 
in lichens is in many cases so close as to appear genetic, and also it often 
happens that as the gonidium multiplies it becomes free from the hypha. 

In his Meuioire sur les Lichens, Tulasne 2 described the colourless 
filaments as being fungal in appearance. The green cells he recognized as 
organs of nutrition, and once and again in his paper he states that they 
arose directly by a sort of budding process from the medullary or cortical- 
filaments, either laterally or at the apex. This apparently reasonable view 
of their origin was confirmed by other writers on the subject: by Speer- 
schneider 3 in his account of the anatomy of Usnea barbata, by de Bary 4 , 
and by Schwendener 5 in their earlier writings. But even while de Bary 
accepted the hyphal origin of the gonidia, he noted 6 that, accompanying 
Opegrapha atra and other Graphideae, on the bark were to be found free 
Chroolepus cells similar to the gonidia in the lichen thallus. He added that 
gonidia of certain other lichens in no way differed from Protococcus cells; 
and as for the gelatinous lichens he declared that "either they were the 
perfect fruiting form of Nostocaceae and Chroococcaceae hitherto looked 
on as algae or that these same Nostocaceae and Chroococcaceae are algbe 
which take the form of Col/etna, Ephebe, etc., when attacked by an ascomy- 
cetous fungus." 

All these investigators, and other lichenologists such as Nylander 7 , still 
regarded the free-living organisms identified by them as similar to the green 
cells of the thallus, as only lichen gonidia escaped from the matrix and 
vegetating in an independent condition. 

The old controversy has in recent years been unexpectedly reopened by 
Elfving 8 who has sought again to prove the genetic origin of the green cells. 
His method has been to examine a large series of lichens by making 
sections of the growing areas, and he claims to have observed in every case 

1 Bayrhoffer 1851. 2 Tulasne 1852. ;! Speerschneider 1854. 4 de Bary 1866, p. 242. 

* Schwendener 1860, p. 125. 6 deBary 1866, p. 291. ~ Nylander 1870. 8 Elfving 1903 and 1913. 



26 CONSTITUENTS OF THE LICHEN THALLUS 

the hyphal origin of the gonidia: not only of Cystococcus but also of Trente- 
potdia, Stigonema and Nostoc. In the case of Cystococcus, the gonidium, he 
says, arises by the swelling of the terminal cell of the hypha to a globose 
form, and by the gradual transformation of the contents to a chlrophyll- 
green colour, with power of assimilation. In the case of filamentous gonidia 
such as Trentepohlia, the hyphal cells destined to become gonidia are 
intercalary. In Peltigera the cells of the meristematic plectenchyma become 
transformed to blue-green Nostoc cells. 

A study was also made by him of the formation of cephalodia 1 , the 
gonidia of which differ from those of the " host" thallus. In Peltigera aphtkosa 
he claims to have traced the development of these bodies to the branching 
and mingling of the external hairs which, in the end, form a ball of inter- 
woven hyphae. The central cells of the ball are then gradually differentiated 
into Nostoc cells, which increase to form the familiar chains. Elfving allows 
that the gonidia mainly increase by division within the thallus, and that they 
also may escape and live as free organisms. His views are unsupported by 
direct culture experiments which are the real proof of the composite nature 
of the thallus. 

E. MlCROGONlDIA 

Another attempt to establish a genetic origin for lichen gonidia was made 
by Minks 2 . He had found in his examination of Leptogium myochroum that 
the protoplasmic contents of the hyphae broke up into a regular series of 
globular corpuscles which had a greenish appearance. These minute bodies, 
called by him microgonidia, were, he states, at first few in number, but 
gradually they increased and were eventually set free by the mucilaginous 
degeneration of the cell wall. As free thalline gonidia, they increased in 
size and rapidly multiplied by division. Minks was at first enthusiastically 
supported by Miiller 3 who had found from his own observations that micro- 
goiiidia might be present in any of the lichen hyphae and in any part of 
thf thallus, even in the germinating tube of the lichen spore, and was in that 
case most easily seen when the spores germinated within the ascus. He 
argued that as spores originated within the ascus, so microgonidia were 
developed within the hyphae. Minks's theories were however not generally 
accepted and were at last wholly discredited by Zukal 4 who was able to 
prove that the greenish bodies were contracted portions of protoplasm in 
hyphae that suffered from a lowered supply of moisture, the green colour 
not being due to any colouring substance, but to light effect on the pro- 
teins an outcome of special conditions in the vegetative life of the plant. 
Darbishire 5 criticized Minks's whole work with great care and he has arrived 
at the conclusion that the microgonidium may be dismissed as a totally 
mistaken conception. 

1 See p. .33. - Minks 1878 and 1879. 3 Muller 1878 and 1884. 4 Zukal 1884. 5 Darbishire 1895!. 



LICHEN GONIDIA 27 



F. COMPOSITE NATURE OF THALLUS 

Schwendener 1 meanwhile was engaged on his study of lichen anatomy. 
Though at first he adhered to the then accepted view of the genetic con- 
nection between hyphae and gonidia, his continued examination of the 
vegetative development led him to publish a short paper 2 in which he 
announced his opinion that the various blue-green and green gonidia were 
really algae and that the complete lichen in all cases represented a fungus 
living parasitically on an alga: in Ephebe, for example, the alga was a form 
of Stigonema, in the Collemaceae it was a species of Nostoc. In those lichens 
enclosing bright green cells, the gonidia were identical with Cystococcus 
humicola, while in Graphideae the brightly coloured filamentous cells were 
those of Chroolepus (Trentepohlia). This statement he repeated in an 
appendix to the larger work on lichens 3 and again in the following year 4 
when he described more fully the different gonidial algae and the changes 
produced in their structure and habit by the action of the parasite: "though 
eventually the alga is destroyed," he writes, "it is at first excited to more 
vigorous growth by contact with the fungus, and in the course of generations 
may become changed beyond recognition both in size and form." In support 
of his theory of the composite constitution of the thallus, Schwendener 
pointed out the wide distribution and frequent occurrence in nature of the 
algae that become transformed to lichen gonidia. He claimed as further 
proof of the presence of two distinct organisms that, while the colourless 
filaments react in the same way as fungi on the application of iodine, the 
gonidia take the stain of algal membranes. 

G. SYNTHETIC CULTURES 

Schwendener's "dual hypothesis," as it was termed, excited great interest 
and no little controversy, the reasons for and against being debated with 
considerable heat. Rees 5 was the first who attempted to put the matte*. to 
the proof by making synthetic cultures. For this purpose he took spores 
from the apothecium of a Collema and sowed them on pure cultures of Nostoc, 
and as a result obtained the formation of a lichen thallus, though he did not 
succeed in producing any fructification. He observed further that the 
hyphal filaments from the germinating spore died off when no Nostoc was 
forthcoming. 

Bornet 6 followed with his record of successful cultures. He selected for 
experiment the spores of PJiyscia (Xanthoria) parietina and was able to 
show that hyphae produced from the germinating spore adhered to the free- 

1 Schwendener 1860, etc. 2 Schwendener 1867. 3 Schwendener 1868, p. 195. 
4 Schwendener 1869. 5 Rees 1871. 6 Bornet 1872. 



CONSTITUENTS OF THE LICHEN THALLUS 



growing cells of Protococcus 1 viridis and formed the early stages of a lichen 
thallus. Woronin 2 contributed his observations on the gonidia of Parmelia 
(Physcid) pulverulenta which he isolated from the thallus and cultivated in 
pure water. He confirmed the occurrence of cell division in the gonidia and 
also the formation of zoospores, these again forming new colonies of algae 
identical in all respects with the thalline gonidia. He was able to see the 
germinating tube from a lichen spore attach itself to a gonidium, though he 
failed in his attempts to induce further growth. In our own country Archer 3 
welcomed the new views on lichens, and attempted cultures but with very 
little success. Further synthetic cultures were made by Bornet 4 , Treub 5 and 
Borzi 6 with a series of lichen spores. They also were able to observe the 
first stages of the thallus. Borzi observed spores of Physcia (Xanthorid) 
parietina scattered among Protococcus cells on the branch of a tree. The 
spores had germinated and the first branching hyphae had already begun to 
encircle the algae. 

Additional evidence in favour of the theory of the independent origin of 
the colourless filaments and the green cells was furnished by Stahl's 7 re- 
search on hymenial gonidia in Endocarpon (Fig. 5). By making synthetic 




Endocarpon pusillum 
edw. Asci and spores, 
with hymenial gonidia x 
320 (after Stahl). 




Fig. 6. Endocarpon pusillum Hedw. Spore 
germinating in contact with hymenial 
gonidia x 320 (after Stahl). 



1 The authors quoted have been followed in their designation of the various green algae that form 
lichen gonidia. It is however now recognized ( Wille 1913) that either Protococcus viridis Ag. , Chlorella 
or other Protococcaceae may form the universal green coating on trees, etc., and be incorporated as 
lichen gonidia. Pleurococcus vulgaris Naeg. and Pleurococcus Naegeli Chod. are synonyms of Proto- 
coccus virtdis. In that alga there is no pyrenoid, and no zoospores are formed. 

The genus Cystococcus, according to Chodat ( 1913), is characterized by the presence of a pyrenoid 
and by reproduction with zoospores and is identical with Pleurococcus z^fcawMenegh. (non Naeg.), 
though Wille regards Meneghini's species as of mixed content. Paulson and Hastings (1920) now 
find that Chodat's pyrenoid is the nucleus of the cell. 

Woronin ,8 7 a. Archer ,873, ,874, 1875. Bornet r8 73 and 1874. 

'Treub ,873. 'Borzi ,875. 7 Stahl lg 



LICHEN GONIDIA 



29 



cultures of the mature spores with these bodies, he was able to observe not 
only the germination of the spores and the attachment of the filaments to 
the gonidia (Fig. 6), but also the gradual building up of a complete lichen 
thallus to the formation of perithecia and spores. 

Some years later Bonnier 1 made an interesting series of synthetic cultures 
between the spores of lichens germinated in carefully sterilized conditions, 
and algae taken from the open (Figs. 7 and 8). Separate control cultures of 




Fig. 7. Germination of spore of Physcia parietitia De Not. in 
contact with Protococcus viridis Ag. x 950 (after Hornet). 







Fig. 8. Physcia parietitia De Not. Vertical section of thallus 
obtained by synthetic culture x 130 (after Bonnier). 

spores and algae were carried on at the same time, with the result that in 
one case lichen hyphae alone, in the other algae were produced. The various 
lichen spores with which he experimented were sown in association with the 
following algae: 

(i) PROTOCOCCUS. 

Pure synthetic cultures of Physcia ( Xanthoria) parietina were begun in 
August 1884 on fragments of bark. In October 1886 the thallus was several 
centimetres in diameter, and some of the lobes were fruited. 

Physcia stellaris was also grown on bark; in one case both thallus and 
apothecia were developed. 

1 Bonnier 1886 and 1889. 



3 o CONSTITUENTS OF THE LICHEN THALLUS 

Parmelia acetabulum, another corticolous species, formed only a minute 
thallus about 5 mm. in diameter, but entirely identical with normally growing 
specimens. 

(2) PLEUROCOCCUS. 

Lecanora (Rinodina) sophodes, sown on rock in 1883, reached in 1886 a 
diameter of 13 mm. with fully developed apothecia. 

Lecanora ferruginea and L. subfusca after three years' culture formed 
sterile thalli only. 

Lecanora coilocarpa in four years, and L. caesio-rufa in three years formed 
very small thalli without fructification. 

(3) TRENTEPOHLIA (Chroolepus). 

Opegrapha vulgata in two years had developed thallus and apothecia. 
The control culture of the spores formed, as in nature, a considerable felt of 
mycelium in the interstices of the bark, but no pycnidia or apothecia. 

Graphis elegans. Only the beginning of a differentiated thallus was 
obtained with this species. 

Verrucaria muralis (?) T gave in less than a year a completely developed 
thallus. 

Bonnier also attempted cultures with species of Collema and Ephebe, but 
was unsuccessful in inducing the formation of a lichen plant. 

H. HYMENIAL GONIDIA 

Reference has already been made to the minute green cells which were 
originally described by Nylander- as occurring in the perithecia of a few 
Fyrenolichens as free gonidia, i.e. unentangled with lichen hyphae. Fuisting 3 
found them in the perithecium of Polyblastia (Staurothele) catalepta at a very 
early stage of its development when the perithecial tissues were newly 
differentiated from those of the surrounding thallus. The gonidia enclosed 
in the perithecium differed in no wise from those of the thallus: they had 
become mechanically enclosed in the new tissue; and while those in the 
outer compact layers died off, those in the centre of the structure, where a 
hollow space arises, were subject to very active division, becoming smaller 
in the process and finally filling the cavity. Winter's 4 researches on similar 
lichens confirmed Fuisting's conclusions: he described them as similar to 
the thalline gonidia but- lighter in colour and of smaller size, measuring 
frequently only 2-3 ^ in diameter, though this size increased to about 7 yu, 
when cultivated outside the perithecium. 

Stahl 5 sufficiently demonstrated the importance of these gonidia in 

1 Bonnier was probably experimenting with an Arthopyrenia. Verrucaria species combine with 
Protococcus or according to Chodat with Coccobotrys gen. nov. 

2 Nylander [858. * Fuisting 1868, p. 674. * Winter 1876, p. 264. 5 Stahl 1877. 



LICHEN GONIDIA 31 

supplying the germinating spores with the necessary algae. They come to 
lie in vertical rows between the asci and, owing to pressure, assume an 
elongate form 1 (Figs. 5 and 6). They have been seen in very few lichens, in 
Endocarpon and Staurothele, both rather small genera of Pyrenolichens,and,so 
far as is known, in two Discolichens, Lecidea pkylliscocarpa and L.phyllocaris, 
the latter recorded from Brazil by Wainio 2 , and, on account of the inclusion 
of gonidia in the hymenium, placed by him in a section, Gonothecium, 

I. NATURE OF ASSOCIATION BETWEEN ALGA AND FUNGUS 

a. CONSORTIUM AND SYMBIOSIS. These cultures had established con- 
vincingly the composite nature of the lichen thallus, and Schwendener's 
opinion, that the relationship between the two organisms was some 
varying degree of parasitism, was at first unhesitatingly accepted by most 
botanists. Reinke 3 was the first to point out the insufficiency of this view 
to explain the long continued healthy life of both constituents, a condition 
so different from all known instances of the disturbing or fatal parasitism of 
one individual on another. He recognized in the association a state of 
mutual growth and interdependence, that had resulted in the production 
of an entirely new type of plant, and he suggested Consortium as a truer 
description of the connection between the fungus and the alga. This term 
had originally been coined by his friend Grisebach in a paper 3 describing 
the presence of actively growing Nostoc algae in healthy Gunnera stems; 
and Reinke compared that apparently harmless association with the similar 
phenomenon in the lichen thallus. The comparison was emphasized by him 
in a later paper 4 on the same subject, in which he ascribes to each "consort" 
its function in the composite plant, and declares that if such a mutual life 
of Alga and Ascomycete is to be regarded as one of parasitism, it must be 
considered as reciprocal parasitism; and he insists that "much more 
appropriate for this form of organic life is the conception and title of Con- 
sortium" In a special work on lichens, Reinke 5 further elaborated his theory 
of the physiological activity and mutual service of the two organisms forming 
the consortium. 

Frank H suggested the term Homobium as appropriate, but it' is faulty 
inasmuch as it expresses a relationship of complete interdependence, and 
it has been proved that the fungus partly, and the alga entirely, have the 
power of free growth. 

A wider currency was given to this view of a mutually advantageous 
growth by de Bary 7 . He followed Reinke in refusing to accept as satisfactory 
the theory of simple parasitism, and adduced the evident healthy life of the 
algal cells the alleged victims of the fungus as incompatible with the 

1 See p. 62. 2 Wainio 1890, 2, p. 29. :! Reinke 1872, p. 108. 

4 Reinke 187.^. 5 Reinke 1873'-, p. 98. 6 Frank 1876. 7 de Bary 1879. 



3 2 CONSTITUENTS OF THE LICHEN THALLUS 

parasitic condition. He proposed the happily descriptive designation of a 
Symbiosis or conjoint life which was mostly though not always, nor in equal 
degree, beneficial to each of the partners or symbionts. 

b. DIFFERENT FORMS OF ASSOCIATION. The type of association be- 
tween the two symbionts varies in different lichens. Bornet 1 , in describing 
the development of the thallus in certain members of the Collemaceae, 
found that though as a rule the two elements of the thallus, as in some 
species of Collema itself, persisted intact side by side, there was in other 
members of the genus an occasional parasitism: short branches from the 
main hyphae applied themselves by their tips to some cell of the Nostoc 
chain (Fig. 9). The cell thus seized upon began to increase in size, and the 




Fig. 9. Pkysma chalazanum Arn. Cells of Nostoc chains penetrated 
and enlarged by hyphae x 950 (after Bomet). 

plasma became granular and gathered at the side furthest away from the 
point of attachment. Finally the contents were used up, and nothing was 
left but an empty membrane adhering to the fungus hypha. In another 
species the hypha penetrated the cell. These instances of parasitism are 
most readily seen towards the edge of the thallus where growth is more 
active; towards the centre the attached cells have become absorbed, and 
only the shortened broken chains attest their disappearance. The other 
cells of the chains remain uninjured. 

In Synalissa, a small shrubby gelatinous genus, the hypha, as described 
by Bornet and by Hedlund 2 , pierces the outer wall of the gelatinous alga 
(Gloeocapsd) and swells inside to a somewhat globose haustorium which 
rests in a depression of the plasma (Fig. 10). The alga, though evidently 



Bornet 1873. 



2 Hedlund 1892. 



LICHEN GONIDIA 



undamaged, is excited to a division which takes place on a plane that passes 
through the haustorium; the two daughter-cells then separate, and in so 
doing free themselves from the hypha. 

Hedlund followed the process of association between the two organisms 
in the lichens Micarea (Biatorina) prasina and 
M. denigrata {Biatorina synothea), crustaceous 
species which inhabit trunks of trees or palings. 
In these the alga, one of the Chlorophyceae, has 
assumed the character of a Gloeocapsa but on 
cultivation it was found to belong to the genus 
Gloeocystis. The cells are globose and rather 
small ; they increase by the division of the con- 
tents into two or at most four portions which 
become rounded off and covered with a mem- 
brane before they become free from the mother- 
cell. The lichen hypha, on contact with any one 
of the green cells, bores through the outer membrane and swells within to a 
haustorium, as in the gonidia of Synalissa. 

Penetrating haustoria were demonstrated by Peirce 1 in his study of the 
gonidia of Ramalina reticulata. In the first stage the tip of a hypha had 
pierced the outer wall of the alga, causing the protoplasm to contract away 
from the point of contact (Fig. 11). More advanced stages showed the 
extension of the haustorium into the centre of the cell, and, finally, the 




Fig. 10. Synalissa symphorea Nyl. 
Algae (Gloeocapsa) with hyphae 
from the internal thallus x 480 
(after Hornet). 




Fig. 1 1 . Gonidia from Ramalina reticulata 
Nyl. A,gonidium pierced and cell con- 
tents shrinking x 560 ; B, older stage, 
the contents of gonidium exhausted x 900 
(after Peirce). 




Fig. 12. Pertusaria globultfera Nyl. Fungus 
and gonidia from gonidial zone x 500 
(after Darbishire). 



complete disappearance of the contents. In many cases it was found that 
penetration equally with clasping of the alga by the filament sets up an 
irritation which induces cell-division, and the alga, as in Synalissa, thus 
becomes free from the fungus. Hue 2 has recorded instances of penetration 
in an Antarctic species, Physcia puncticulata. It is easy, he says, to see the tips 
of the hyphae pierce the sheath of the gonidium and penetrate to the nucleus. 

1 Peirce 1899. z Hue 1915. 

S.L 3 



34 CONSTITUENTS OF THE LICHEN THALLUS 

Lindau 1 has described the association between fungus and alga in 
Pertusaria and other crustaceous forms as one of contact only (Fig. 12). 
He found that the cell-membrane of the two adhering organisms was un- 
broken. Occasionally the algal cell showed a slight indentation, but was 
otherwise unchanged. The hyphal branch was somewhat swollen at the tip 
where it 'touched the alga, and the wall was slightly thinner. The attach- 
ment between the two cells was so close, however, that pressure on the cover- 
glass failed to separate them. 

Generally the hypha simply surrounds the gonidium with clasping 
branches. Many algae also lie free in the gonidial zone, and Peirce 2 claims 
that these are larger, more deeply coloured and in every way healthier 
looking than those in the grasp of the fungus. He ignores, however, the case 
of the soredial algae which though very closely invested by the fungus are 
yet entirely healthy, since on their future increase depends in many cases 
the reproduction of new individual lichens. 

In a recent study of a crustaceous sandstone lichen, " Caloplaca pyracea" 
Claassen 3 has sought to prove a case of pure parasitism. The rock was at first 
covered with the green cells of Cystococcus sp. Later there appeared greyish- 
white patches on the green, representing the invasion of the lichen fungus. 
These patches increased centrifugally, leaving in time a bare patch in the 
centre of growth which was again colonized by the green alga. The lichen 
fruited abundantly, but wherever it encroached the green cells were more 
or less destroyed. The true explanation seems to be that the green cells 
were absorbed into the lichen thallus, though enough of them persisted to 
start new colonies on any bare piece of the stone. In the same way large 
patches of Trentepohlia aurea have been observed to be gradually invaded 
by the dark coloured hyphae of Coenogonium ebeneum. In time the whole of 
the alga is absorbed and nothing is to be seen but the dark felted lichen. 
The free alga as such disappears, but it is hardly correct to describe the 
process as one of destruction. 

This algal genus Trentepohlia (Chroolepus) forms the gonidia of the 
Graphideae, Roccelleae, etc. It is a filamentous aerial alga which increases 
by apical growth. In the Graphideae, many of which grow on trees beneath 
the outer bark (hypophloeodal), the association between the two symbionts 
may be of the simplest character, but was considered by Frank 4 to be of an 
advanced type. According to his observations and to those of Lindau 5 , the 
fungal hyphae penetrate first between the cells of the periderm. The alga, 
frequently Trentepohlia nmbrina, tends to grow down into any cracks of the 
surface. It goes more deeply in when preceded by the hyphae. In some 
species both organisms maintain their independent growth, though each 
shows increased vigour when it conies into contact with the other. In some 

1 Lindau 1895'. * Peirce I899 



LICHEN GONIDIA 35 

instances the cells of the alga are clasped by the fungus which causes the 
disintegration of the filament. The cells lose their bright yellow or reddish 
colour and are changed in appearance to greenish lichen gonidia; but no 
penetration by haustoria has ever been observed in Trentepohlia. 

Bachmann's 1 study of a similar gonidium in a calcicolous species of 
Opegrapha confirms Frank's results. The algae had pierced not only between 
the looser lime granules but also through a crystal of calcium carbonate, and 
occupied nests scooped out in the rock by means of acid formed and excreted 
by their filaments. When association took place with the fungus, the algal 
cells were more restricted to a gonidial zone; but some of the cells, having 
been pushed aside by the hyphae, had started new centres of gonidia. On 
contact with the hyphae there was a tendency to bud out in a yeast-like 
growth. 

In the thallus of the Roccelleae, the algal filament, also a Trentepohlia, is 
broken up into separate cells, but in the Coenogoniaceae, whether the 
gonidium be a Cladophora as in Racodium,or a Trentepohlia as in Coenogonium, 
the filaments remain intact and are invested more or less closely by the 
hyphae. 

A somewhat different type of association takes place between alga and 
fungus in Strigula complanata, an epiphyllous lichen more or less common 
in tropical regions. Cunningham 2 , who found it near Calcutta, described the 
algal constituent and placed it in a new genus, Mycoidea (Cephaleuros). It 
forms small plate-like expansions on the surface of the leaf, and also pene- 
trates below the cuticle, burrowing between that and the epidermal cells; 
occasionally, as observed by Cunningham, rhizoid-like growths pierce deeper 
into the tissue into and below the epidermal layer. Very frequently, in the 
wet season, a fungus takes possession of 
the alga and slender colourless hyphae 
creep along its surface by the side of the 
cell rows, sending out branches which 
grow downwards. Marshall Ward 3 de- 
scribed the same lichen from Ceylon. He 
states that the alga may be attacked at 
any stage, and if it is in a very young con- 
dition it is killed by the fungus; at a Fg- '3- Outer edge of Phycopeltis expansa 
. . Term., the alga attacked by hyphae and 

more advanced period of growth it COn- passing into separate gonidia x 500 (after 
tinues to develop as an integral part of Vaughan Jennings), 
the lichen thallus, but with more frequently divided and smaller cells. 
Vaughan Jennings 4 observed Strigula complanata in New Zealand associated 
with a closely allied chroolepoid alga Phycopeltis expansa. He also noted the 
growth of the fungus over the alga breaking up the plates of tissue and 

1 Bachmann 1913. 2 Cunningham 1879. 3 Ward 1884. 4 Jennings 1895. 

32 




36 CONSTITUENTS OF THE LICHEN THALLUS 

separating the cells which, from yellow, change to a green colour and 
become rounded off (Fig. 13). The mature lichen, a white thallus dotted 
with black fruits, contrasts strikingly with the yellow membranous alga. 
Lichen formation usually begins near the edge of the leaf and the margin of 
the thallus itself is marked by a green zone showing where the fungus has 
recently come into contact with the alga. 

More recently Hans Fitting 1 has described " Mycoidea parasitica" as it 
occurs on evergreen leaves in Java. The alga, a species of Cephaleuros, 
though at first an epiphyte, becomes partially parasitic at maturity. It pene- 
trates below the cuticle to the outer epidermal cells and may even reach 
the tissue below. When it is joined by the lichen fungus, both constituents 
grow together to form the lichen. Fitting adds that the leaf is evidently but 
little injured. In this lichen the alga in the grip of the fungus loses its 
independence and may be killed off: it is an instance of something like 
intermittent parasitism. 

J. RECENT VIEWS ON SYMBIOSIS AND PARASITISM 

No hyphal penetration of the bright-green algal cell by means of 
haustoria had been observed by the earlier workers, Bornet 2 , Bonnier 3 and 
others, though they followed Schwendener 4 in regarding the relationship as 
one of host and parasite. Lindau, also, after long study accepted parasitism 
as the only adequate explanation of the associated growth, though he never 
found the fungus actually preying on the alga. 

In recent years interest in the subject has been revived by the researches 
of Elenkin 5 , a Russian botanist who claims to have established a case for 
parasitism or rather "endosaprophytism." He has demonstrated by means 
of staining reagents the presence in the thallus of large numbers of dead 
algal cells. A few empty membranes are to be found in the cortex and in 
the gonidial zone, but the larger proportion occur below the gonidial zone 
and partly in the medulla. He describes the lower layer as a "necral" or 
"hyponecral" zone, and he considers that the hyphae draw their nourishment 
chiefly from dead algal material. The fungus must therefore be regarded in 
this case as a saprophyte rather than a parasite. The algae, he considers, 
may have perished from want of sufficient light and air or they may have 
been destroyed by an enzyme produced by the fungus. The latter he thinks 
is the more probable, as dead cells are frequently present among the living 
algae of the gonidial zone. To the action of the enzyme he also attributes 
the angular deformed appearance of many gonidia and the paler colour and 
gradual disintegration of their contents which are finally used up as endo- 
saprophytic nourishment by the fungus. Dead algal cells were more easily 

1 Fitting 1910. 2 Bornet 1873. 3 Bonnier i88 9 2 . 4 Schwendener 1867. 

5 Elenkin 1903! and 1904!, 19042. 



LICHEN GONIDIA 37 

seen, he tells us, in crustaceous lichens associated with " Pleurococcus" or 
" Cystococcus" \ they were much less frequent in the larger foliose or fruticose 
lichens. Dead cells of Trentepohlia were also difficult to find. 

In a second paper Elenkin records one clear instance of a haustorium 
entering an algal cell, and says he found some evidence of hyphal branches 
penetrating otherwise uninjured gonidia, round holes being visible in their 
outer wall, but he holds that it is the cell-wall of the alga that is mainly 
dissolved by the ferment and then used as food by the hyphae. 

No allowance has been made by Elenkin for the normal wasting common 
to all organic beings: the lichen fungus is continually being renewed, 
especially in the cortical structures, and the alga must also be subject to 
change. He 1 claims, nevertheless, that his observations have proved that the 
one symbiont is always preying on the other, either as a parasite or as a 
saprophyte. He has likened the conception of symbiosis to that of a balance 
between two organisms, "a moveable equilibrium of the symbionts." If, he 
says, we could conceive a state where the conditions of life would be equally 
favourable for both partners there would be true mutualism, but in practice 
one only is favoured and gains the upper hand, using its advantage to prey 
on the other. Unless the balance is redressed, the complete destruction of 
the weaker is certain, and is followed in time by the death of the stronger. 
The fungus being the dominant partner, the balance, he considers, is tipped 
in its favour. 

Elenkin's conclusions are not borne out by the long continued and healthy 
life of the lichen. There is no record of either symbiont having succumbed 
to the other, and the alga, when set free, is unchanged and able to resume its 
normal development. Without the alga the fungus cannot form the ascigerous 
fruit. Is that because as a parasite within the lichen it has degenerated past 
recovery, or has it become so adapted to symbiosis that in saprophytic con- 
ditions it fails to develop ? 

Another Russian lichenologist, U. N. Danilov 2 , records results which 
would seem to support the theory of parasitism. He found that from the 
clasping hyphae minute haustoria were produced, which penetrate the algal 
cell-wall, and branch when within the outer membrane, thus forming a 
delicate network over the plasma; secondary haustoria arising from this 
network protrude into the interior and rob the cell-contents. He observed 
gonidia filled with well-developed hyphae and these, after having exhausted 
one cell, travel onwards to others. Some gonidia under the influence of the 
fungus had become deformed and were finally killed. As a proof of this 
latter statement he adduces the presence in the thallus of some gonidia 
containing shrivelled protoplasm, of others entirely empty. He considers, as 
further evidence in favour of parasitism, the finding of empty membranes as 

1 Elenkin I9o6 2 . 2 Danilov 1910. 



5G01.1 



38 CONSTITUENTS OF THE LICHEN THALLUS 

well as of colourless gonidia filled with the hyphal network. This description 
hardly tallies with the usual healthy appearance of the gonidial zone in the 
normal thallus, and it has been suggested that where the fungus filled the 
algal cell, it was as a saprophyte preying on dead material. 

The gradual perishing of algal cells in time by natural decay and their 
subsequent absorption by the fungus is undisputed. It is open to question 
whether the varying results recorded by these workers have any further 
significance. 

These observations of Elenkin and Danilov have been proved to be 
erroneous by Paulson and Somerville Hastings 1 . They examined the thalli 
of several lichens (Xanthoria parietina, Cladonia sp., etc.) collected in early 
spring when vegetative growth in these plants was found to be at its highest 
activity. They found an abundant increase of gonidia within the thallus, 
which they regarded as sporulation of the algae, and the most careful methods 
of staining failed to reveal any case of penetration of the gonidia by the 
hyphae. 

Nienburg 2 has published some recent observations on the association of 
the symbionts. In the wide cortex of a Pertusaria he found not only the 
densely compact hyphae, but also isolated gonidia. In front of these latter 
there was a small hollow cavity and, behind, parallel hyphae rich in contents. 
These gonidia had originated from the normal gonidial zone. They were 
moved upward by special hyphae called by Nienburg "push-hyphae." After 
their transportation, the algae at once divide and the products of division 
pass to a resting stage and become the centre of a new thalline growth. A 
somewhat similar process was noted towards the apex of Evernia furfur acea. 
Radial hyphae pushed up the cortex, leaving a hollow space over the gonidial 
zone. Into the space isolated algae were thrust by "push-hyphae." In this 
lichen he also observed the penetration of the algal cell by haustoria of the 
fungus. He considers that the alga reaps advantage but also suffers harm, 
and he proposes the term helotism to express the relationship. 

An instructive case of the true parasitism of a fungus on an alga has been 
described by Zukal 3 in the case of Endomyces scytonemata which he calls 
a "half-lichen." The mature fungus formed small swellings on the filaments 
of the Scytonema and, when examined, the hyphae were seen to have attacked 
the alga, penetrating the outer gelatinous sheath and then using up the 
contents of the green cells. It is only after the alga has been destroyed and 
absorbed, that asci are formed by the fungus. Zukal contrasts the develop- 
ment of this fungus with the symbiotic growth of the two constituents in 
EpJiebe where both grow together for an indefinite time. 

Mere associated growth however even between a fungus and an alga 
does not constitute a lichen. An instance of such growth is described by 
1 Paulson and Hastings 1920. 2 Nienburg 1917. 3 Zuka j l89I> 



LICHEN GONIDIA 39 

Sutherland ] in an account of marine microfungi. One of these, a species of 
Mycosphaerella, was found on Pelvetia canaliculata, and Sutherland claims 
that as no apparent injury was done to the alga, it was a case of 
symbiosis and that there was formed a new type of lichen. The mycelium, 
always intercellular, pervaded the whole host-plant, and the fungal fruits 
were invariably formed on the algal receptacles close to the oogonia. Their 
position there is, of course, due to the greater food supply at that region. 
Both fungus and alga fruited freely. A closer analogy could have been found 
by the writer in the smut fungus which grows with the host-cereal until 
fruiting time; or with the mycorrhiza of Calluna which also pervades every 
part of the host-plant without causing any injury. In the true lichen, the 
alga, though constituting an important part of the vegetative body, takes no 
part in reproduction, except by division and increase of the vegetative cells 
within the thallus. The fruiting bodies are always of a modified fungal 
nature. 

2. PHYSIOLOGY OF THE SYMBIONTS 

The occurrence of isolated cases of parasitisVn the fungus preying on 
the alga in any case leaves the general problem unsolved. The whole 
question turns on the physiological activity and requirements of the two 
component elements of the thallus. From what sources do they each 
procure the materials essential to them as living organisms? It is chiefly 
a question of nutrition. 

A. NUTRITION OF ALGAE 

a. CHARACTER OF ALGAL CELLS. Gonidia are chlorophyll-containing 
bodies and assimilate carbon-dioxide from the atmosphere by photo- 
synthesis as do the chlorophyll cells of other plants. They also require 
water and mineral salts which, in a free condition, they absorb from their 
immediate surroundings, but which, in the lichen thallus, they must obtain 
from the fungal hyphae. If the nutriment supplied to them in their inclosed 
position be greater or even equal to what the cells could procure as free- 
living algae, then they undoubtedly gain rather than lose by their asso- 
ciation with the fungus, and are not to be considered merely as victims of 
parasitism. 

b. SUPPLY OF NITROGEN. Important contributions on the subject of 
algal nutrition have been made by Beyerinck 2 and Artari 3 . The former 
conducted a series of culture experiments with green algae, including the 
gonidia of Physcia (Xanthoria) parietina. He successfully isolated the 
lichen gonidia and, at first, attempted to grow them on gelatine with an 
infusion of the Elm bark from which he had taken the lichen. Growth was 

1 Sutherland 1915. 2 Beyerinck 1890. 3 Artari 1902. 



40 CONSTITUENTS OF THE LICHEN THALLUS 

very slow and very feeble until he added to the culture- medium a solution 
of malt-extract which contains peptones and sugar. Very soon he obtained 
an active development of the gonidia, and they multiplied rapidly by 
division 1 as in the lichen thallus. This proved to him conclusively the great 
advantage to the algae of an abundant supply of nitrogen. 

Artari in his work has demonstrated that there are two different physio- 
logical races of green algae: (i) those that absorb peptones which he 
designates peptone-algae and (2) those that do not so absorb peptones. 
He tested the cells of Cystococcus humicola taken from the thallus of Physcia 
parietina, and found that they belonged to the peptone group and were 
therefore dependent on a sufficiency of nitrogenous material to attain their 
normal vigorous growth. It was also discovered by Artari that the one 
race can be made by cultivation to pass over to the other: that ordinary 
algae can be educated to live on peptones, and peptone-algae to do without. 

We learn further from Beyerinck's researches that Ascomycetes, the 
group of fungi from which the hyphae of most lichens are derived, are 
what he terms ammonia^sugar fungi; that is to say, the hyphae can 
abstract nitrogen from ammonia salts and, with the addition of sugar, can 
form peptones. The lichen peptone-algae are thus evidently, by their 
contact with such fungi, in a favourable position for securing the nitro- 
genous food supply most suited to their requirements. In their deep-seated 
layers, they are to a large extent deprived of light, but it has been proved 
by Artari 2 in a series of culture experiments extending over a long period, 
that the gonidia of Xanthoria parietina remain green in the dark under 
very varied conditions of nutriment, though the colour is distinctly fainter. 

Recently Treboux 3 has revised the work done by Artari and Beyerinck 
in reference to Cystococcus humicola. He denies that two physiological races 
are represented in this alga, the lichen gonidia, in regard to the nitrogen 
that they absorb, behaving exactly as do the free-living forms of the species. 
He finds that the gonidium is not a peptone-carbohydrate organism in the 
sense that it requires nitrogen in the form of peptones, inorganic ammonia 
salts being a more acceptable food supply. Treboux concludes that his 
results favour the view that the gonidia are in an unfavourable situation for 
receiving the kind of nitrogenous compound most advantageous to them, 
that .they are therefore in a sense "victims" of parasitism, though he 
qualifies the condition as being a lichen-parasitism or helotism. This view 
does not accord with Chodat's 4 results: in his cultures of gonidia he 
observed that with glycocoll or peptone, which are nearly equivalent, they 
developed four times better than with potassium nitrate as their nitrogenous 
food, and he concluded that they assimilated nitrogen better from bodies 
allied to peptides. 

1 See p. 56. 2 Artari 1902. 3 Treboux 1912. 4 Chodat 1913. 



LICHEN. GONIDIA 41 

c. EFFECT OF SYMBIOSIS ON THE ALGA. Treboux's observations how- 
ever convinced him that the alga leads but a meagre existence within the 
thallus. Cell-division the expression of active vitality was, he held, of rare 
occurrence in the slowly growing lichen-plant, and zoospore formation in 
entire abeyance. He contrasts this sluggish increase 1 with the rapid multi- 
plication of the free-living algal cells which cover whole tree-trunks with 
their descendants in a comparatively short time. These latter cells, he 
finds, are indeed rather smaller, being generally the products of recent 
division, but mixed with them are numbers of larger resting cells, com- 
parable in size with the lichen gonidia. He states further, that the gonidia 
are less brightly green and, as he judges, less healthy, though in soredial 
formation or in the open they at once regain both colour and power of 
division. Treboux had entirely failed to observe the sporulation which is so 
abundant at certain seasons. 

Their quick recovery seems also a strong argument in favour of the 
absolutely normal condition of metabolism within the gonidial cell; and 
the paler appearance of the chlorophyll is doubtless associated with the 
acquisition of carbohydrates from other' sources than by photosynthesis. 
There is a wide difference between any degree of unfavourable life-conditions 
and parasitism however slight, even though the balance of gain is on the 
side of the fungus. It is not too fanciful to conclude that the demand for 
nitrogen on the part of the alga has influenced its peculiar association with 
the fungus. In the thallus of hypophloeodal lichens it has been proved 
indeed that the alga Trentepohlia with apical growth is an active agent in 
the symbiotic union. Cystococctis and other green algal cells are stationary, 
but they are doubtless equally ready for as many of them are equally 
benefited by the association. Keeble 2 has pointed out in the case of 
Convoluta roscoffensis that nitrogen-hunger induces the green algae to 
combine forces with an animal organism, though the benefit to them is only 
temporary and though they are finally sacrificed. The lichen gonidia, on 
the contrary, persist for a long time, probably far beyond their normal 
period of existence as free algae. 

Examples of algal association with other plants might be cited here: of 
Nostoc in the roots of Cycas and in the cells of Anthoceros, and of Anabczna 
in the leaf-cells of Azolla, but in these instances it is generally held that 
the alga secures only shelter. It was by comparing the lichen-association 
with the harmless invasion of Gunnera cells by Nostoc that Reinke 3 arrived 
at his conception of "consortism." 

d. SUPPLY OF CARBON. Carbon, the essential constituent of all organic 
life, is partly drawn from the carbon-dioxide of the air, and assimilated by 

1 See Paulson and Hastings 1920. 2 Keeble 1910. 3 Reinke 1872. 



42 CONSTITUENTS OF THE LICHEN THALLUS 

the green cells; it is also partly contributed by the fungus as a product of 
its metabolism. A proof of this is afforded by Dufrenoy 1 : he found a 
Parmelia growing closely round pine needles and even sending suckers into 
the stomata. He covered the lichen with a black cloth and after seven weeks 
found that the gonidia had remained very green. That growth had not 
been checked was evidenced by an unusual development of soredia and 
of spermogonia. Dufrenoy describes the condition as a parasitism of the 
algae on the fungus which in turn was drawing nourishment from the 
pine needles. 

Artari 2 has proved that lichen gonidia can obtain carbohydrates from 
the substratum as well as by photosynthesis. He cultivated the gonidia of 
Xanthoria parietina and Placodium murorum on media which contained 
organic substances as well as mineral salts, while depriving them of atmo- 
spheric carbon-dioxide and in some cases of light also. The gonidia not 
only grew well but, even in the dark, they remained normally green, a 
phenomenon coinciding with Etard and Bouilhac's 3 experience in growing 
Nostoc in the dark: with suitable culture media the alga retained its colour. 
Nostoc also grows in the dark in the rhizome of Gunnera. Radais' 4 experi- 
ments with Chlorella vulgaris confirmed these results. On certain organic 
media growth and cell-division were as rapid in the dark as in the light, 
and chlorophyll was formed. The colour was at first yellowish and the full 
green arrived slowly, especially on sugar media, but in ten days it was 
uniform and normal. 

When making further experiments with the alga, Siichococcus badllaris, 
Artari 5 found that it also grew well on an organic medium and that grape 
sugar was the most valuable carbonaceous food supply. Chodat 6 also found 
that sugar or glucose was a desirable ingredient of culture media. 

Treboux 7 , in his work on organic acids, has also proved by experimental 
cultures with a large series of algae, including the gonidia of Peltigera, that 
these green plants in the absence of light and in pure cultures would grow 
and form carbohydrates if the culture medium contained a small percentage 
of organic acids. The acids he employed were combined with potassium 
and were thus rendered neutral or slightly alkaline; acetate of potash 
proved to be the most advantageous compound of any that was tested. 
Amino-acids and ammonia salts were added to provide the necessary 
nitrogen. Oxalic acid and other organic acids of varying composition are 
peculiarly abundant in lichen tissues and may be a source of carbon supply. 
Marshall Ward 8 has found calcium carbonate crystals in the lower air- 
containing tissues of Strigula complanata. 

Treboux finally concluded from his researches that just as fungi can 

1 Dufrenoy 19 i 8. 2 Artari 1899. Etard and Bouilhac 1898. 4 Radais 1900. 

6 Artari 1901. Chodat 1913. ? Treboux 1905. 8 Marshall Ward 1884. 



LICHEN GONIDIA 43 

extract carbohydrates from many sources, so algae can secure their carbon 
supply in a variety of ways. He affirms that the metabolic activity of the 
alga in these cultural conditions is entirely normal, and the various cell- 
contents are formed as in the light. Whether, in this case, starch is formed 
directly from the acids or through a series of combinations has not been 
determined. Uhlir 1 , with electric lighting, made successful cultures of 
Nostoc isolated from Collemaceae on silicic acid, proving thereby that these 
gonidia do not require a rich nutriment. A certain definite humidity was 
however essential, and bacteria were never eliminated as they are associated 
with the gelatinous membranes of Nostocaceae. 

e. NUTRITION WITHIN THE SYMBIOTIC PLANT. Culture experiments 
bearing more directly on the nutrition of lichens as a whole were carried 
out by F. Tobler 2 . He proved that the gonidia had undoubtedly drawn on 
the calcium oxalate secreted by the hyphae for their supply of carbon. In 
a culture medium of poplar-bark gelatine he grew hyphae of Xantkoria 
parietina, and noted an abundant deposit of oxalate crystals on their cell- 
walls. A piece of the lichen thallus including both symbionts and grown on 
a similar medium formed no crystals, and microscopic examination showed 
that crystals were likewise absent from the hyphae of the thallus that had 
grown normally on the tree, the inference being that the gonidia used them up 
as quickly as they were deposited. It must be remembered in this connection, 
however, that Zopf 3 has stated that where lichen acids are freely formed 
as, for instance, in Xanthoria parietina, there is always less formation and 
deposit of calcium oxalate crystals, which may partly account for their 
absence in the normal thallus so rich in parietin. 

Tobler next introduced lichen gonidia into a culture medium in which 
the isolated hyphal constituent of a thallus had been previously cultivated, 
and placed the culture in the dark. In these circumstances he found that 
the gonidia were able to thrive but formed no colour: they were obtaining 
their carbohydrates, he decided, not from photosynthesis, but from the 
excretory products such as calcium oxalate that had been deposited in the 
culture medium by the lichen hyphae. We may conclude with more or less 
certainty that the loss of carbohydrates, due to the partial deprivation of 
light and air suffered by the alga owing to its position in the lichen thallus, 
is more than compensated by a physiological symbiosis with the fungus 4 . 
It has indeed been proved that in the absence of free carbon-dioxide, algae 
may utilize the half-bound CO 2 of carbonates, chiefly those of calcium and 
magnesium, dissolved in water. 

/ AFFINITIES OF LICHEN GONIDIA. Chodat 5 has, in recent years, 
made cultures of lichen gonidia with a view to discovering their relation to 

1 Uhlir 1915. 2 Tobler 1911. 3 Zopf 1907. * Chambers 1912. 5 Chodat 1913 



44 CONSTITUENTS OF THE LICHEN THALLUS 

free-living algae and to testing at the same time their source of carbon 
supply. He has come to the conclusion that lichen gonidia are probably in 
no instance the normal Protococcus viridis: they differ from that alga in the 
possession of a pyrenoid and in their reproduction by zoospores when free. 

Careful cultures were made of different Cladonia gonidia which were 
morphologically indistinguishable, and which varied in size from 10 to 16/4 
in diameter, though smaller ones were always present. He recognized them 
to be species of Cystococcus: they have a pyrenoid 1 in the centre and a 
disc-like chromatophore more or less starred at the edge. These gonidia 
grew well on agar, still better on agar-glucose, but best of all with an 
addition of peptone to the culture. There was invariably at first a slight 
difference in form and colour in the mass between the gonidia of one 
species and those of another, but as growth continued they became alike. 

In testing for carbon supply, he found that gonidia grew slowly without 
sugar (glucose), and that, as sources of carbon, organic acids could not 
entirely replace glucose though, in the dark, the gonidia used them to some 
extent; the colony supplied with potassium nitrate, and grown in the dark, 
had reached a diameter of only 2 mm. in three months. With glucose, it 
measured 5 mm. in three weeks, while in three months it formed large 
culture patches. 

A further experiment was made to test their absorption of peptones by 
artificial cultures carried out both in the light and the dark. The gonidia 
grew poorly in all combinations of organic nitrogen compounds. When 
combined with glucose, growth was at once more vigorous though only half 
as much in the dark as in the light, the difference in this respect being 
especially noticeable in the gonidia from Cladonia pyxidata. He concludes 
that as gonidia in these cultures are saprophytic, so in the lichen thallus 
also they are probably more or less saprophytic, obtaining not only their 
nitrogen in organic form but also, when possible, their carbon material as 
glucose or galactose from the hyphal symbiont which in turn is saprophytic 
on humus, etc. 

B. NUTRITION OF FUNGI 

Fungi being without chlorophyll are always indebted to other organisms 
for their supply of carbohydrates. There has never therefore been any 
question as to the advantage accruing to the hyphal constituent in the 
composite thallus. The gonidia, as various workers have proved, have also 
a marked preference for organized nourishment, and, in addition, they obtain 
carbon by photosynthesis. Chodat 2 considers that probably they are thus 
able to assimilate carbon-dioxide in excess, a distinct advantage to the 
hyphae. In some instances the living gonidium is invaded and the contents 
1 See note Paulson and Hastings, p. 28. 2 Chodat 1913. 



LICHEN GONIDIA 45 

used up by the fungus and any dead gonidia are likewise utilized for food 
supply. It is also taken for granted that the fungus takes advantage of the 
presence of humus whether in the substratum or in aerial dust. In such 
slow growing organisms, there is not any large demand for nourishment on 
the part of the hyphae: for many lichens it seems to be mere subsistence 
with a minimum of growth from year to year. 

C. SYMBIOSIS OF OTHER PLANTS 

The conception of an advantageous symbiosis of fungi with other plants 
has become familiar to us in Orchids and in the mycorhizal formation on 
the roots of trees, shrubs, etc. Fungal hyphae are also frequent inhabitants 
of the rhizoids of hepatics though, according to Gargeaune 1 , the benefit to 
the hepatic host-plant is doubtful. 

An association of fungus and green plant of great interest and bearing 
directly on the question of mutual advantage has been described by 
Servettaz 2 . In his study of mosses, he was able to confirm Bonnier's 3 
account of lichen hyphae growing over such plants as Vaticheria and 
the protonema of mosses, which is undoubtedly hurtful; but he also found 
an association of a moss with one of the lower fungi, Streptothrix or 
Oospora, which was distinctly advantageous. In separate cultivation the 
fungus developed compact masses and grew well in peptone agar broth. 

Cultures of the moss, Phascum cuspidatum, were also made from the 
spores on a glucose medium. The specimens in association with the fungus 
were fully grown in two months, while the control cultures, without any 
admixture of the fungus, had not developed beyond the protonema stage. 
Servettaz draws attention to the proved fact that, in certain instances, 
plants benefit when provided with substances similar to their own decay 
products, and he considers that the fungus, in addition to its normal gaseous 
products, has elaborated such substances, as acid products, from the glucose 
medium to the great advantage of the moss plant. 

A symbiotic association of Nostoc with another alga, described by 
Wettstein 4 , is also of interest. The blue-green cells were lodged in the 
pyriform outgrowths of the siphoneous alga, Botrydium pyriforme Kiitz., 
which the author of the paper places in a new genus, Geosiphon. The 
sheltering Nostoc symbioticum fills all of the host left vacant by the plasma, 
and when the season of decay sets in, it forms resting spores which migrate 
into the rhizoids of the host, so that both plants regenerate together. 

Wettstein has compared this symbiotic association with that of lichens, 
and finds the analogy all the more striking in that the membrane of his new 
alga had become chitinous, which he thinks may be due to organic nutrition. 

1 Gargeaune 1911. 2 Servettaz 1913. 3 See p. 65. 4 Wettstein 1915. 



46 CONSTITUENTS OF THE LICHEN THALLUS 

II. LICHEN HYPHAE 

A. ORIGIN OF HYPHAE 

Lichen hyphae form the ground tissue of the thallus apart from the 
gonidia or algal cells. They are septate branched filaments of single cell 
rows and are colourless or may be tinged by pigments or lichen acids to 
some shade of yellow, brown or black. They are of fungal nature, and are 
produced by the mature lichen spore. 

The germination of the spore was probably first observed by Meyer 1 . 
His account of the actual process is somewhat vague, and he misinterpreted 
the subsequent development into thallus and fruit entirely for want of the 
necessary magnification; but that he did succeed in germinating the spores 
is unquestionable. He cultivated them on a smooth surface and they grew 
into a "dendritic formation" a true hypothallus. Many years later the 
development of hyphae from lichen spores was observed by Holle 2 who saw 
and figured the process unmistakably in Borrera (Physcici) ciliaris. 

A series of spore cultures was undertaken by Tulasne 3 with the twofold 
object of discovering the exact origin of hyphae and gonidia and of their 
relationship to each other. The results of his classical experiment with the 
spores of Verrucaria muralis as interpreted by him were accepted by the 
lichenologists of that time as conclusive evidence of the genetic origin of the 
gonidia within the thallus. 

The spores- of the lichen in large numbers had been sown by Tulasne 
in early spring on the smooth polished surface of a piece of limestone, and 




Fig. 14. Germinating spores of Verrucaria nmralis Ach. after two 
months' culture x ca. 500 (after Tulasne). 

were covered with a watch-glass to protect them from dust, etc. At 

irregular intervals they were moistened with water, and from time to time 

1 Meyer 1825. * Ho ,, e ^ , ^^ ^.^ 



LICHEN HYPHAE . 47 

a few spores were abstracted from the culture and examined microscopically. 
Tulasne observed that the spore did not increase or change in volume in the 
process of germination, but that gradually the contents passed out into the 
growing hyphae, till finally a thin membrane only was left and still persisted 
after two months (Fig. 14). For a considerable time there was no septation ; 
at length cross-divisions were formed, at first close to the spore, and then 
later in the branches. The hyphae meanwhile increased in dimension, the 
cells becoming rounder and somewhat wider, though always more slender 
than the spore which had given rise to them. In time a felted tissue was 
formed with here and there certain cells, filled with green colouring matter, 
similar to the gonidia of the lichen and thus the early stages at least of a 
new thallus were observed. The green cells, we now know, must have gained 
entrance to the culture from the air, or they may have been introduced with 
the water. 

B. DEVELOPMENT OF LICHENOID HYPHAE 

Lichen hyphae are usually thick-walled, thus differing from those of fungi 
generally, in which the membranes, as a rule, remain comparatively thin. 
This character was adduced by the so-called "autonomous" school as a proof 
of the fundamental distinction between the hyphal elements of the two 
groups of plants. It can, however, easily be observed that, in the early 
stages of germination, the lichen hyphae, as they issue from the spore, are 
thin-walled and exactly comparable with those of fungi. Growth is apical, 
and septation and branching arise exactly as in fungi, and, in certain circum- 
stances, anastomosis takes place between converging filaments. But if algae 
are present in the -culture the peculiar lichen characteristics very soon 
appear. 

Bonnier 1 , who made a large series of synthetic cultures, distinguishes 
three types of growth in lichenoid hyphae (Fig. 15): 

1. Clasping filaments, repeatedly branched, which attach and surround 
the algae. 

2. Filaments with rather short swollen cells which ultimately form the 
hyphal tissues of cortex and medulla. 

3. Searching filaments which elongate towards the periphery and go to 
the encounter of new algae. 

In five days after germination of the spores, the clasping hyphae had 
laid hold of the algae which meanwhile had increased by division; the 
swollen cells had begun to branch out and ten days later a differentiation 
of tissue was already apparent. The searching filaments had increased in 
number and length, and anastomosis between them had taken place when 

1 Bonnier i88q 2 . 



48 CONSTITUENTS OF THE LICHEN THALLUS 

no further algae were encountered. The cell-walls of the swollen hyphae 
and their branches had begun to thicken and to become united to form a kind 
of cellular tissue or "paraplectenchyma 1 ." At a later date, about a month 




Fig. 15. Synthetic culture of Physcia parietina spores and Protococcus 
viridis five days after germination, s, lichen-spore ; a, septate fila- 
ments ; b, clasping filaments; c, searching filaments, x 500 (after 
Bonnier). 

after the sowing of the spores, there was a definite cellular cortex formed 
over the thallus. The hyphal cells are uninucleate, though in the medulla 
they may be i-2-nucleate. 

The hyphae in close contact with the gonidia remain thin- walled, and 
have been termed by Wainio 2 "meristematic." They furnish the growing 
elements of the lichen either apical or intercalary. In most genera the organs 
of fructification take rise from them, or in their immediate neighbourhood, 
and isidia and soredia also originate from these gonidial hyphae. 

As the filaments pass from the gonidial zone to other layers, the cell- 
walls become thicker with a consequent reduction of the cell-lumen, very 
noticeable in the pith, but carried to its furthest extent in the "decomposed" 
cortex where the cells in the degenerate tissue often become reduced to dis- 
connected streaks indicating the cell-lumen, and the outer cortical layer is 
merely a continuous mass of mucilage. 

All lichen -tissues arise from the branching and septation of the hyphae, 
the septa always forming at right angles to the long axis of the filaments. 
There is no instance of longitudinal cell-division except in the spores of 
certain genera (Collema, Urceolaria, Polyblastia, etc.). The branching of the 
hypha is dichotomous or lateral, and very irregular. Frequent septation and 
coherent growth result in the formation of plectenchyma. 

1 Term coined by Lindau (1899) to describe the pseudo-cellular tissue of lichens and fungi now 
referred to as "plectenchyma." 2 Wainio 1897. 



LICHEN HYPHAE 49 

C. CULTURE OF HYPHAE WITHOUT GONIDIA 

Artificial cultures had demonstrated the germination of lichen spores, 
with the formation of hyphae, and from synthetic cultures of fungus and 
alga complete lichen plants had been produced. To Moller 1 we owe the first 
cultures of a thalline body from the fungus alone, both from spermatia and 
from ascospores. The germination of the spermatia has a direct bearing 
on their function as spores or as sexual organs and is described in a 
later chapter. 

The ascospores of Lecanora sitbfusca were caught in a drop of water on 
a slide as they were ejaculated from the ascus, and, on the following day, a 
very fine germinating tube was seen to have pierced the exospore. The 
hypha became slightly thicker, and branching began on the third day. If 
in water alone the culture soon died off, but in a nutrient solution growth 
slowly continued. The hyphae branched out in all directions from the spore 
as a centre and formed an orbicular expansion which in fourteen days had 
reached a size of 'I mm. in diameter. After three weeks' growth it was large 
enough to be visible without a lens ; the mycelial threads were more crowded, 
and certain terminal hyphae had branched upwards in an aerial tuft, this 
development taking place from the centre outwards. Moller marked this 
stage as the transition from a mere protothallus to a thallus formation. In 
three months a diameter of i'5-2 mm. was reached; a transverse section 
gave a thickness of "86 mm. and from the under side loose hyphae branched 
downwards and attached the thallus, when it had been transferred to a solid 
substratum such as cork. Above these rhizoidal hyphae, a stratum of rather 
loose mycelium represented the medulla, and, surmounting that, a cortical 
layer in which the hyphae were very closely compacted. Delicate terminal 
branches rose into the air over the whole surface, very similar in character 
to hypothallic hyphae at the margin of the thallus. 

Lecanora subfusca has a rather small simple spore; it emitted germinating 
tubes from each end, and a septum across the middle of the spore appeared 
after germination had taken place. Another experiment was with a much 
larger muriform spore measuring 80^, in length and 20 //, in thickness. On 
germination about 20 tubes were formed, some of them rising into the air at 
once, the others encircling the spore, so that the thallus took form imme- 
diately; growth in this case also was centrifugal. In three months a diameter 
of 6 mm. was reached with a thickness of I to 2 mm. and showing a differen- 
tiation into medulla and cortex. The hyphae did not increase in width, but 
frequently globose or ovate swellings arose in or at the ends, a character which 
recurs in the natural growth of hyphae both of lichens and of Ascomycetes. 
These swellings depend on the nutrition. 

1 Moller 1887. 



50 CONSTITUENTS OF THE LICHEN THALLUS 

Pertusaria communis possesses a very large simple spore, but it is multi- 
nucleate and germinates with about 100 tubes which reach their ultimate 
width of 3 to 4 /x before they emerge from the exospore. The hyphae 
encircle the spore, and an opaque thalline growth is quickly formed from 
which rise terminal hyphal branches. In ten weeks the differentiation into 
medulla and cortex was reached, and in five months the hyphal thallus 
measured 4 mm. in diameter and i to 2 mm. in thickness. 

Moller instituted a comparison between the thalli he obtained from the 
spores and those from the spermatia of another crustaceous lichen, Buellia 
punctiformis (B. myriocarpa). After germination had taken place the hyphae 
from the spermatia grew at first more quickly than those from the ascospores, 
but as soon as thallus formation began the latter caught up and, in eight 
weeks, both thalli were of equal size. 

Another comparative culture with the spermatia and ascospores of 
Opegrapha subsiderella gave similar results: the spores of that species are 
elongate-fusiform and 6- to 8-septate; germination took place from the end 
cells in two to three days after sowing. The germinating hyphae corre- 
sponded exactly with those from the spermatia and growth was equally slow 
in both. The middle cells of the spores may also produce germinating tubes, 
but never more than about five were observed from any one spore. A 
browning of the cortical layer was especially apparent in the hyphal culture 
from another lichen, Graphis scripta: a clear brown colour gradually changing 
to black appeared about the same period in all the cultures. 

The hyphae from the spores of Arthonia developed quickest of all: the 
hyphae were very slender, btt in three to four months the growth had reached 
a diameter of 8 mm. In this plant there was the usual outgrowth of delicate 
hyphae from the surface; no definite cortical layer appeared, but only a very 
narrow line of more closely interwoven somewhat darker hyphae. Frequently, 
from the surface of the original thallus, excrescences arose which were the 
beginnings of further thalli. 

Tobler 1 experimenting with Xanthoria parietina gained very similar 
results. The spores were grown in malt extract for ten days, then transferred 
to gelatine. In three to five weeks there was formed an orbicular mycelial 
felt about 3 mm. in diameter and 2 mm. thick. The mycelium was frequently 
brownish even in healthy cultures, but the aerial hyphae which, at first, rose 
above the surface were always colourless. After these latter disappeared a 
distinct brownish tinge of the thallus was visible. In seven months it had 
increased in size to 15 mm. in length, 7 mm. in width and 3 mm. thick with 
a differentiation into three layers: a lower rather dense tissue representing 
the pith, above that a layer of loose hyphae where the gonidial zone would 

1 Tobler 1909. 



LICHEN HYPHAE 51 

normally find place, and above that a second compact tissue, or outer cortex, 
from which arose the aerial hyphae. The culture could not be prolonged 
more than eight months. 

D. CONTINUITY OF PROTOPLASM IN HYPHAL CELLS 

Wahrlich 1 demonstrated that continuity of protoplasm was as constant 
between the cells of fungi as it has been proved to be between the cells of 
the higher plants. His researches included the hyphae of the lichens, Cla- 
donia fimbriata and Physcia (Xanthoria) parietina. 

Baur 2 and Darbishire 3 found independently that an open connection 
existed between the cells of the carpogonial structures in the lichens they 
examined. The subject as regards the thalline hyphae was again taken up 
by Kienitz-Gerloff 4 who obtained his best results in the hypothecial tissue 
of Peltigera canina and P. polydactyla. Most of the cross septa showed one 
central protoplasmic strand traversing the wall from cell to cell, but in some 
instances there were as many as four to six pits in the walls. The thickening 
of the cell-walls is uneven and projects variously into, the cavity of the cell. 
Meyer's 5 work was equally conclusive: all the cells of an individual hypha, 
he found, are in protoplasmic connection ; and in plectenchymatous tissue 
the side walls are frequently perforated. Cell-fusions due to anastomosis are 
frequent in lichen hyphae, and the wall at or near the point of fusion is also 
traversed by a thread of protoplasm, though such connections are regarded 
as adventitious. Fusions with plasma connections are numerous in the 
matted hairs on the upper surface of Peltigera canina and they also occur 
between the hyphae forming the rhizoids of that lichen. The work of Salter 6 
may also be noted. He claimed that his researches tended to show complete 
anatomical union between all the tissues of the lichen plant, not only between 
the hyphae of the various tissues but also between hyphae and gonidia. 

III. LICHEN ALGAE 

A. TYPES OF ALGAE 

The algal constituents of the lichen thallus belong to the two classes, 
Myxophyceae, generally termed blue-green algae, and Chlorophyceae which 
are coloured bright-green or yellow-green. Most of them are land forms, 
and, in a free condition, they inhabit moist or shady situations, tree-trunks, 
walls, etc. They multiply by division or by sporulation within the thallus; 
zoospores are never formed except in open cultivation. The determination 
of the genera and species to which the lichen algae severally belong is often 
uncertain, but their distribution within the lichen kingdom is as follows: 

1 Wahrlich 1893. 2 Baur 1898. 3 Darbishire 1899. 

4 Kienitz-Gerloff 1902. 5 Meyer 1902. 6 Salter 1902. 

42 



CONSTITUENTS OF THE LICHEN THALLUS 



a. MYXOPHYCEAE ASSOCIATED WITH PHYCOLICHENS. The blue-green 
algae are characterized by the colour of their pigments which persists 
in the gonidial condition giving various tints to the component lichens, and 
by the gelatinous sheath in which most of them are enclosed. This sheath, 
both in the lichen gonidia 1 and in free-living forms, imbibes and retains 
moisture to a remarkable extent and the thallus containing blue-green algae 
profits by its power of storing moisture. Myxophyceae form the gonidia 
of the gelatinous lichens as well as of some other non-gelatinous genera. 
Several families are represented 2 : 

Fam. CHROOCOCCACEAE. This family includes unicellular algae with 
thick gelatinous sheaths. They increase normally by division, and colonies 
arise by the cohesion of the cells. Several genera form gonidia: 

1. CHROOCOCCUS Naeg. Solitary or forming small colonies of 2-4-8 
cells (Fig. 1 6) generally surrounded by firm gelatinous colourless sheaths in 
definite layers (lamellate). Chroococcus is considered by some lichenologists 
to form the gonidium of Cora, a genus of Hymenolichens. 

2. MlCROCYSTis Kiitz. Globose or subglobose cells forming large 
colonies surrounded by a common gelatinous layer (gonidia of Coris- 
cium). 

3. GLOEOCAPSA Kiitz. (including Xanthocapsd). Globose cells with a 







wm 

WB 



Fig. 17. Gloeocapsa magma 
Kiitz. x 450 (after West). 



Fig. 16. Examples of Chroococcus. A, Ch. gigantens 
West ; B, Ch. turgidus Naeg. ; C and D, Ch. schizo- 
dermaticus West x 450 (after West). 

lamellate gelatinous wall, forming colonies enclosed in a common sheath 
(Fig. 17); the inner integument is often coloured red or orange. These 

1 Nylander (1866) gave the term "gonimia" to the blue-green algae of the Phycolichens, retaining 
the term " gonidia " for the bright-green algae of the Archilichens : the distinction is not now main- 
tained. 

2 For further details see also the chapter on Classification. 



LICHEN ALGAE 



53 
Gloeocapsa 



two genera form the gonidia in the family Pyrenopsidaceae. 
polydermatica Kiitz. has been identified as a lichen gonidium. 

Fam. NOSTOCACEAE. Filamentous algae unbranched and without base 
or apex. 

NOSTOC Vauch. Composed of flexuous trichomes, with intercalary 
heterocysts (colourless'cells) (Fig. 1 8). Dense gelatinous colonies of definite 




Fig. 18. Examples of Nostoc. N. Linckia Born. A, nat. size ; B, small portion x 340 ; 
C, N. coerulescens Lyngbye, nat. size (after West). 




Fig. 19. Example of Scytnnema alga. 5. mirabile Thur. C, apex of a branch ; D, organ 
of attachment at base of filament." x 440 (after West). 

form are built up by cohesion. In some lichens the trichomes retain their 
chain-like appearance, in others they are more or less broken up and massed 
together, with disappearance of the gelatinous sheath (as in Peltigera); 
colour mostly dark blue-green. 

Nostoc occurs in a few or all of the genera of Pyrenidiaceae, Collemaceae, 
Pannariaceae, Peltigeraceae and Stictaceae, and N. sphaericum Vauch 



54 



CONSTITUENTS OF THE LICHEN THALLUS 



(N. lichenoides Kutz.) has been determined as the lichen gonidium. When 
the chains are broken up it has been wrongly classified as another alga, 
Poly coccus punctiformis. 

Fam. SCYTONEMACEAE. Trichomes of single-cell rows, differentiated into 
base and apex. Pseudo-branching arises at right angles to the main filament. 

SCYTONEMA Ag. Pseudo-branches piercing the sheath and passing out 
as twin filaments (Fig. 19); colour, golden-brown. This alga occurs in 
genera of Pyrenidiaceae, Ephebaceae, Pannariaceae, Heppiaceae, in Petractis 
a genus of Gyalectaceae, and in Dictyonema one of the Hymenolichens. 

Fam. STIGONEMACEAE. Trichomes of several-cell rows with base and 
apex ; colour, golden-brown. 

STIGONEMA Ag. Stouter than Scytonema, with transverse and vertical 
division of the cells, and generally copious branching (Fig. 20). This alga 
occurs only in a few genera of Ephebaceae. S.panniforme Kirchn. (Siro- 
siphon pulvinatus Breb.) has been determined as forming the gonidium. 

Fam. RIVULARIACEAE. Trichomes with a heterocyst at the base and 
tapering upwards, enclosed in mucilage (Fig. 21). 




Fig. 20. Stigonema sp. x 200 (after 
Comere). 




-**-* 1 

Fig. 21. Examples of Rivularia ; A, B, ,R.Bia- 
sokttiana Menegh. ; D and E, R. minutula 
Born, and Fl. A and D nat. size; B, C and E 
x 4 8o (after West). 



LICHEN ALGAE 



55 



RlVULARiA Thuret. In tufts fixed at the base and forming roundish 
gelatinous colonies; colour, blue-green. The gonidium of Lichinaceae has 
been identified as R. nitida Ag. 

Algae belonging to one or other of these genera of Myxophyceae also 
combine with the hyphae of Archilichens to form cephalodia 1 and Krem- 
pelhuber 2 has recorded and figured a blue-green alga, probably Gloeocapsa, 
in Baeomyces paeminosus from the South Sea Islands. They also form the 
gonidia in a few species and genera of such families as Stictaceae and 
Peltigeraceae. 

b. CHLOROPHYCEAE ASSOCIATED WITH ARCHILICHENS. The lichens of 
this group are by far the most numerous both in genera and species, though 
fewer algal families are represented. 

Fam. PROTOCOCCACEAE. Consisting of globular single cells, aggregated 
in loose colonies, dividing variously. 

i. PROTOCOCCUS VIRIDIS Ag. (Pleurococcus vulgaris Menegh., Cystococ- 
cushumicola Naeg.). Cells dividing 
into 2, 4 or 8 daughter-cells and 
not separating readily; in exces- 
sive moisture forming short fila- 
ments. The cells contain parietal 
chloroplasts, and, according to 
Chodat 3 , are without a pyrenoid 
(Fig. 22). This alga, and allied 
species, forms the familiar green 
coating of tree-trunks, walls etc., 
and, in lichenological literature, 
are quoted as the gonidia of most 
of the crustaceous foliose and fru- 
ticose lichens. Chodat 3 , who has 
recently made comparative artificial cultures of algae, throws doubt on the 
identity of many such gonidia. He lays great emphasis on the presence or 
absence of a pyrenoid in algal cells. West, on the contrary, considers the 
pyrenoid as an inconstant character. Chodat insists that the gonidia that 
contain pyrenoids belong to another genus, Cystococcus Chod. (iwn Naeg.), 
a pyrenoid-containing alga, which, in addition to multiplying by division 
of the cells, also forms spores and zoospores when cultivated. He further 
records the results of his cultures of gonidia, and finds that those taken 
from closely related lichens, such as different species of Cladonia, though 
they are alike morphologically, yet show constant variations in the culture 
colonies. These, he holds, are sufficient to indicate difference of race if not 




Fig. 22. Pleurococcus vulgaris Menegh. (Protococ- 
cus viridis Ag. ). chl. chloroplast ; p. protoderma 
stage; /<?, palmelloid stage; py, pyrenoid. x 520 
(after West). 



See p. 133. 



2 Krempelhuber 1873. 



Chodat 1913. 



CONSTITUENTS OF THE LICHEN THALLUS 




Fig. 23. Cystococcus Cladoniae 
pyxidatae Chod. from cul- 
ture x 800 (after Chodat). 



of species and he designates the algae, according to the lichen in which 
they occur, as Cystococcus Cladoniae pyxidatae, C. Cladoniae Jimbriatae, etc. 

Meanwhile Paulson and Somerville Hastings 1 by their careful research 
on the growing thallus have thrown considerable light on the identity of the 
Protococcaceous lichen gonidium. They selected such well-known lichens 
as Xanthoria parietina, Cladonia spp. and others, which they collected 
during the spring months, February to April, the period of most active 
growth. Many of the gonidia, they found, were in a stage of reproduction, 
that showed a simultaneous rounding off of the 
gonidium contents into globose bodies varying in 
number up to 32. Chodat had figured this method 
of "sporulation" in his cultures of the lichen goni- 
dium both in Chlorella Beij. and in Cystococcus Chod. 
(Fig. 23). It has now been abundantly proved that 
this form of increase is of frequent occurrence in the 
thallus itself. Chlorella has been suggested as .prob- 
ably the alga forming these gonidia and recently 
West has signified his acquiescence in this view 2 . 

2. CHLORELLA Beij. Occurring frequently on damp ground, bark of 
trees, etc., dividing into numerous daughter- 
cells, probably reduced zoogonidia (Fig. 23). 

Chodat distinguishes between Cystococcus 
and Chlorella in that Cystococcus may form 
zoospores (though rarely), Chlorella only 
aplanospores. He found three gonidial species, 
Chlorella lichina in Cladonia rangiferina, Ch. 
viscosa and Ch. Cladoniae in other Cladonia 
spp. 

3. COCCOBOTRYS Chod. The cells of this 
new algal genus are smaller than those of 
Cystococcus ox Protococcus and have no pyrenoid. 
They were isolated by Chodat from the thallus 
of Verrucaria nigrescens (Fig. 24), and, as 
they have thick membranes, they adhere in 
a continuous layer or thallus. Chodat also 
claims to have isolated a species of Cocco- 
botrys from Dermatocarpon miniatum, a foliose 
Pyrenolichen. 

4. COCCOMYXA Schmidle. Cells ellipsoid, also without a pyrenoid. 
Two species were obtained by Chodat from the thallus of Solorinae and 
are recorded as Coccomyxa Solorinae croceae and C. Solorinae saccatae. 




'ig. 23 A. A, C, Chlorella vulgaris 
Beyer. B and C, stages in division 
x about 800 (after Chodat) ; E, 
Chi. faginea Wille x 520 (after 
Gerneck); F I Chi. miniata ; F, 
vegetable cell ; G I, formation 
and escape of gonidia x 1000 
(after Chodat). 



1 Paulson and Hastings 1920. 



2 Paulson in litt. 



LICHEN ALGAE 



57 



Coccomyxa subellipsoidea is given 1 as the gonidium of the primitive 
lichen Botrydina vulgaris (Fig. 25). The cells are surrounded by a common 
gelatinous sheath. 



Fig. 24. Coccobotrys Verrucariae Chod. 
from culture x 800 (after Chodat). 




Fig. 25. Coccomyxa subellipsoidea Acton. 
Actively dividing cells, the dark portions 
indicating the chloroplasts x 1000 (after 
Acton). 



5. DiPLOSPHAERA Bial. 2 D. Chodati was taken from the thallus of 
Lecanora tartarea and successfully cultivated. It resembles Protococcus^ but 
has smaller cells and grows more rapidly ; it is evidently closely allied to 
that genus, if not merely a form of it. 

6. IJROCOCCUS Kiitz. Cells more or less globose, rather large, and 
coloured with a red-brown pigment, with the cell-wall thick and lamellate, 
forming elongate strands of cells (Fig. 26). Recorded by Hue 3 in the 
cephalodium of Lepolichen coccopkora, a Chilian lichen. 

Fam. TETRASPORACEAE. Cells in groups of 2 or 4 surrounded by a 
gelatinous sheath. 

i . PALMELLA Lyngb. Cells globose, oblong or ellipsoid, grouped without 
order in a formless mucilage (Fig. 27). Among lichens associated with 
Palmella are the Epigloeaceae and Chrysothricaceae. 





Fig. 26. Urococcus sp. Group of cells 
much magnified (after Hassall). 



Fig. 27. Palmella sp. x 400 (after Comere). 



2. GLOEOCYSTIS Naeg. Cells oblong or globose with a lamellate 
sheath forming small colonies ; colour, red-brown 
(Fig. 28). This alga along with Urococcus was 
found by Hue in the cephalodia of Lepolichen 
coccophora, but whereas Gloeocystis frequently occu- 
pies the cephalodium alone, Urococcus is always 
accompanied by Scytonema, the normal gonidium 
of the cephalodium. 




Fig. 28. Gloeocystis sp. x 400 
(after Comere). 



1 Acton 1909. 



- Bialosuknia 1909. 



Hue 1905. 



58 CONSTITUENTS OF THE LICHEN THALLUS 




Fig. 30. Example of Cladophora. Cl. glomerata Klitz 
A.nat. s,ze; B, x 85 (after West). 



iff. 29- A, Trentepohlia umbrina Born ; 
K, /. aurea Mart, x 300 (after Kiitz.). 



LICHEN ALGAE 59 

Fam. TRENTEPOHLIACEAE. Filamentous and branched, the filaments 
short and creeping or long and forming tufts and felts or cushions; colour, 
brownish-yellow or reddish-orange. 

TRENTEPOHLIA Born. Branching alternate; cells filled with red or 
orange oil ; no pyrenoids (Fig. 29). A large number of lichens are associated 
with this genus : Pyrenulaceae, Arthoniaceae, Graphidaceae, Roccellaceae, 
Thelotremaceae, Gyalectaceae and Coenogoniaceae, etc., in whole or in part. 
Two species have been determined, T. umbrina Born., the gonidium of the 
Graphidaceae, and T. aurea which is associated with the only European 
Coenogonium, C. ebeneum (Fig. 3). Deckenbach 1 claimed that he had proved 
by cultures that T. umbrina was a growth stage of T. aurea. 

Fam. CLADOPHORACEAE. Filamentous, variously and copiously branched, 
the cells rather large and multinucleate. 

CLADOPHORA Klitz. Filaments branching, of one-cell rows, attached 
at the base ; colour, bright or dark green ; mostly aquatic and marine 
(Fig. 30). Only one lichen, Racodium rupestre, a member of the Coeno- 
goniaceae, is associated with Cladophora. It is a British lichen, and is always 
sterile. 

Fam. MYCOIDEACEAE. Epiphytic algae consisting of thin discs which 
are composed of radiating filaments. 

1. MYCOIDEA Cunningh. (Cephaleuros Kunze). In Mycoidea parasitica 
the filaments of the disc are partly 

erect and partly decumbent, reddish 
to green (Fig. 31). It forms the goni- 
dium of the parasitic lichen, Strigula 
complanata, which was studied by 
Marshall Ward in Ceylon 2 . Zahl- 
bruckner gives Phyllactidium as an 

alternative gonidium of Strigula- Fig. 31. Mycoidea parasitica Cunningh. much 

magnified (after Marshall Ward). 
CG3.C. 

2. PHYCOPELTIS Millard. Disc a stratum one-cell thick, bearing seta, 
adnate to the lower surface of the leaf, yellow-green in colour. Phycopeltis 
(Fig. 32) has been identified as the gonidium of Strigula complanata in 
New Zealand and of Mazosia (Chiodectonaceae), a leaf lichen from tropical 
America. 

1 Deckenbach 1893. 

2 In a comparative study of leaf algae from Ceylon and Barbadoes, N. Thomas (1913) came to the 
conclusion that Marshall Ward's alga in its early stages is the same as Phyllactidium ti'opicum 
Moebius ; and that the Barbadoes alga with which she was working represented the older stages, it 
being then subcuticular in habit, forming rhizoids, barren and sterile aerial hairs and subcuticular 
zoosporangia. 




6o 



CONSTITUENTS OF THE LICHEN THALLUS 



There is some confusion as to the genera of algae that form the gonidia 
of these epiphyllous lichens. Phyllactidium 
given by Zahlbruckner as the gonidium of 
all the Strigulaceae (except Strigula in 
part) is classified by de Toni 1 as probably 
synonymous with Phycopeltis Millard, and 
as differing from Mycoidea parasitica in the 
mode of growth. 

Fam. PRASIOLACEAE. Thallus filamen- 
Fig. 3,. Phycopeltis expansa Jenn. tous, often expanded into broad sheets by 

much magnified (after Vaughan th e fusion of the filaments in one plane. 

Jennings). 

PRASIOLA Ag. Thallus filamentous, of one- to many-cell rows, or 
widely expanded (Fig. 33). The gonidium of Mastoidiaceae (Pyreno- 
carpeae). 






Fig. 33. Prasiola parietina Wille x 500 (after West). 

B. CHANGES INDUCED IN THE ALGA 

a. MYXOPHYCEAE. Though, as a general rule, the alga is less affected 
by its altered life-conditions than the fungus, yet in many instances it 
becomes considerably modified in appearance. In species of the genus 
Pyrenopsis small gelatinous lichens the alga is a Gloeocapsa very similar to 
G. magma. In the open it forms small colonies of blue-green cells surrounded 
by a gelatinous sheath which is coloured red with gloeocapsin. As a 
gonidium lying towards or on the outside of the granules composing the 
thallus, the red sheath of the cells is practically unchanged, so that the 
resemblance to Gloeocapsa is unmistakable. In the inner parts of the thallus, 
the colonies are somewhat broken up by the hyphae and the sheaths are not 

1 De Toni 1889. 



LICHEN ALGAE 61 

only less evident but much more faintly coloured. In Synalissa, a minute 
shrubby lichen which has the same algal constituent, the tissue of the thallus 
is more highly evolved, and in it the red colour can barely be seen and 
then only towards the outside; at the centre it disappears entirely. The 
long chaplets of Nostoc cells persist almost unchanged in the thallus of the 
Collemaceae, but in heteromerous genera such as Pannaria and Peltigera 
they are broken up, or they are coiled together and packed into restricted 
areas or zones. The altered alga has been frequently described as Polycoccus 
punctiformis. A similar modification occurs in many cephalodia, so that the 
true affinity of the alga, in most instances, can only be ascertained after free 
cultivation. 

Bornet 1 has described in Coccocarpia molybdaea the change that the alga 
Scytonema undergoes as the thallus develops : in very young fronds the 
filaments of Scytonema are unchanged and are merely enclosed between 
layers of hyphae. At a later stage, with increase of the thallus in thickness, 
the algal filaments are broken up, their covering sheath disappears, and the 
cells become rounded and isolated. Petractis (Gyalecta) exanthematica has 
also a Scytonema as gonidium, and equally exact observations have been 
made by Funfstiick 2 on the way it is transformed by symbiosis: with the 
exception of a very thin superficial' layer, the thallus is immersed in the 
rock and is permeated by the alga to its lowest limits, 3 to 4 mm. below the 
surface, Petractis being a homoiomerous lichen. The Scytonema trichomes 
embedded in the rock become narrower, and the sheath, which in the 
epilithic part of the thallus is 4/4 wide, disappears almost entirely. The 
green colour of the cells fades and septation is less frequent and less regular. 
The filaments in that condition are very like oil-hyphae and can only be 
distinguished as algal by staining reagents such as alkanna. They never 
seem to be in contact with the fungal elements : there is no visible appearance 
of parasitism nor even of consortism. 

b. CHLOROPHYCEAE. As a rule the green-celled gonidium such as 
Protococcus is not changed in form though the colour may be less vivid, but 
in certain lichens there do occur modifications in its appearance. In Micarea 
(Biatorina) prasina, Hedlund 3 noted that the gonidium was a minute alga 
possessing a gelatinous sheath similar to that of a Gloeocapsa. He isolated 
the alga, made artificial cultures and found that, in the altered conditions, 
it gradually increased in size, threw off the gelatinous sheath and developed 
into normal Protococcus cells, measuring 7 to IO/LI in diameter. The gelatinous 
sheath was thus proved to be merely a biological variation, probably of 
value to the lichen owing to its capacity to imbibe and retain moisture. 
Zukal 4 also made cultures of this alga, but wrongly concluded it was a 
Gloeocystis, 

1 Bornet 1873. 2 Fiinfstiick 1899. 3 Hedlund 1892. 4 Zukal 1895, p. 19. 



62 CONSTITUENTS OF THE LICHEN THALLUS 

Moebius 1 has described the transformation from algae to lichen gonidia 
in a species epiphytic on Orchids in Porto Rico. He had observed that most 
of the leaves were inhabited by a membranaceous alga, Phyllactidium, and 
that constantly associated with it were small scraps of a lichen thallus con- 
taining isolated globose gonidia. The cells of the alga, under the influence 
of the invading fungus, were, in this case, formed into isolated round bodies 
which divided into four, each daughter-cell becoming surrounded by a 
membrane and being capable, in turn, of further division. 

Frank 2 followed the change from a free alga to a gonidium in Chroolepus 
(Trentepohlia) umbrinum, as shown in the hypophloeodal thalli of the 
Graphideae. The alga itself is frequent on beech bark, where it forms wide- 
spreading brownish-red incrustations consisting of short chains occasionally 
branched. The individual cells have thick laminated membranes and vary 
in width from 2Oyu, to 37/1. The free alga constantly tends to penetrate below 
the cortical layers of the tree on which it grows, and the immersed cells 
become not only longer and of a thinner texture, but the characteristic red 
colour so entirely disappears, that the growing penetrating apical cell may 
be light green or almost colourless. As a lichen gonidium the alga under- 
goes even more drastic changes : the red oily granules gradually vanish and 
the cells become chlorophyll-green or, if any retain a bright colour, they are 
orange or yellow. The branching of the chains is more regular, the cells 
more elongate and narrower; usually they are about 13 to 21/1, long and S/j, 
wide, or even less. Deeper down in the periderm, the chains become dis- 
integrated into separate units. Another notable alteration takes place in 
the cell-membrane which becomes thin and delicate. It has, however, been 
observed that if these algal cells reach the surface, owing to peeling of the 
bark, etc., they resume the appearance of a normal Trentepohlia. 

In certain cases where two kinds of algae were supposed to be present 
in some lichens, it has been proved that one species only is represented, the 
difference in their form being caused by mechanical pressure of the sur- 
rounding hyphae, as in Endocarpon and Staurothele where the hymenial 
gonidia are cylindrical in form and much smaller than those of the thallus. 
They were on this account classified by Stahl 8 under a separate algal genus, 
Stichococcus, but they are now known to be growth forms of Protococcus, the 
alga that is normally present in the thallus. Similar variations were found 
by Neubner 4 in the gonidia of the Caliciaceae, but, by culture experiments 
with the gonidia apart from the hyphae, he succeeded in demonstrating 
transition forms in all stages between the " Pleurococcus" cells and those of 
" Stichococcus" though the characters acquired by the latter are transmitted 
to following generations. The transformation from spherical to cylindrical 

i Moebius 1888. 2 Frank 1876, p. 158. 3 Stahl 1877. * Neubner 1893. 



LICHEN ALGAE 63 

algal cells had been also noted by Krabbe 1 in the young podetia of some 
species of Cladonia, the change in form being due to the continued pressure 
in one direction of the parallel hyphae. 

Isolated algal cells have been observed within the cortex of various 
lichens. They are carried thither by the hyphae from the gonidial zone in 
the process of cortical formation, but they soon die off as in that position 
they are deprived of a sufficiency of air and of moisture. Forssell 2 found 
Xanthocapsa cells embedded in the hymenium of Omphalaria Heppii. They 
were similar to those of the thallus, but they were not associated with hyphae 
and had undergone less change than the thalline algae. 

C. CONSTANCY OF ALGAL CONSTITUENTS 

Lichen hyphae of one family or genus, as a rule, combine with the same 
species of alga, and the continuity of genera and species is maintained. 
There are, however, related lichens that differ chiefly or only in the characters 
of the gonidia. Among such closely allied genera or sections of genera may 
be cited Sticta with bright-green algae and the section Stictina with blue- 
gr-een; Peltidea similarly related to Peltigera and Nephroma to Nephromium. 
In the genus S0/orina,some of the species possess bright-green, others blue- 
green algae, while in one, 5. crocea*, there is an upper layer of small bright- 
green gonidia that project in irregular pyramids into the upper cortex ; 
while below these there stretches a more or less interrupted band of blue- 
green Nostoc cells. The two layers are usually separated by strands of 
hyphae, but occasionally they come into close contact, and the hyphal 
filaments pass from one zone to the other. In this genus cephalodia con- 
taining blue-green Nostoc are characteristic of all the "bright-green" species. 
Harmand 4 has recorded the presence of two different types of gonidia in 
Lecanora atra f. subgrumosa\ one of them, the normal Protococcus alga of the 
species, the other, pale-blue-green cells of Nostoc affinity. 

Forssell 5 states that in Lecanora (Psoroma) hypnorum, the normal bright- 
green gonidia of some of the squamules may be replaced by Nostoc. In that 
case they are regarded as cephalodia, though in structure they exactly 
resemble the squamules of Pannaria pezizoides, and Forssell considers that 
there is sufficient evidence of the identity of the hyphal constituent in these 
two lichens, the alga alone being different. 

It may be that in Archilichens with a marked capacity to form a second 
symbiotic union with blue-green algae, a tendency to revert to a primitive 
condition is evident a condition which has persisted wholly in Peltigera 
with its Nostoc zone, but is manifested only by cephalodia formation in the 

1 Krabbe 1891. 2 Forssell 1885. 3 Hue 1910. * Harmand 1913, p. 1050. 

5 Forssell 1886. 



6 4 CONSTITUENTS OF THE LICHEN THALLUS 

Peltidea section of the genus. In this connection, however, we must bear in 
mind Forssell's view that it is the Archilichens that are the more primitive 1 . 
The alien blue-green algae with their gelatinous sheaths are adapted to 
the absorption and retention of moisture, and, in this way, they doubtless 
render important service to the lichens that harbour them in cephalodia. 



D. DISPLACEMENT OF ALGAE WITHIN THE THALLUS 

a. NORMAL DISPLACEMENT. Lindau 2 has contrasted the advancing 
apical growth of the creeping alga Trentepohlia with the stationary condition 
of the unicellular species that multiply by repeated division or by sporulation, 
and thus form more or less dense zones and groups of gonidia in most 
lichens. The fungus in the latter case pushes its way among the algae and 
breaks up the compact masses by a shoving movement, thus letting in light 
and air. The growing hypha usually applies itself closely round an algal 
cell, and secondary branches arise which in time encircle it in a network of 
short cells. In the thallus of Variolaria* the hyphae from the lower tissues, 
termed push-hyphae by Nienburg 4 , push their way into the algal groups and 
filaments composed of short cells come to lie closely round the individual 
gonidia. Continued growth is centrifugal, and the algae are carried outward 
with the extension of the hyphae (Fig. 12). Cell-division is more active at the 
periphery, that being the area of vigorous growth, and the algal cells are, in 
consequence, generally smaller in that region than those further back, the 
latter having entered more or less into a resting condition, or, as is more 
probable, these smaller cells are aplanospores not fully mature. 

b. LOCAL DISPLACEMENT. Specimens of Parmelia physodes were found 
several times by Bitter, the grey-green surface of which was marbled with 
whitish lines, caused by the absence of gonidia under these lighter-coloured 
areas. The thallus was otherwise healthy as was manifested by the freely 
fruiting condition : no explanation of the phenomenon was forthcoming. 
Bitter compared the condition with the appearance of lighter areas on the 
thallus of Parmelia obscurata. 

Something of the same nature was observed on the thallus of a Peltigera 
collected by F. T. Brooks near Cambridge. The marking took the form of 
a series of concentric circles, starting from several centres. The darker lines 
were found on examination to contain the normal blue-green algal zone, 
while the colour had faded from the lighter parts. The cause of the difference 
in colouration was not apparent. 

1 See Chap. VII. 2 Lindau 1895. 3 Darbishire 1897. 4 Nienburg 1917. 



LICHEN ALGAE 



E. NON-GONIDIAL ORGANISMS ASSOCIATED WITH LlCHEN HYPHAE 

Bonnier 1 made a series of cultures with lichen spores and green cells 
other than those that form lichen gonidia. In one instance he substituted 
Protococcus botryoides for the normal gonidia of Parmelia (Xanthoria) 
parietina\ in another of his cultures he replaced Protococcus viridis by the 
filamentous alga Trentepolilia abietina. In both cases the hyphae attached 
themselves to the green cells and a certain stage 
of thallus formation was reached, though growth 
ceased fairly early. Another experiment made 
with the large filaments of Vaucheria sessilis met 
with the same amount of success (Fig. 34). The 
germinating hyphae attached themselves to the 
alga and grew all round it, but there was no ad- 
vance to tissue formation. 

Cultures were also made with the protonema 
of mosses. Either spores of mosses and lichens 
were germinated together, or lichen spores were 
sown in close proximity to fully formed proto- 
nemata. The developing hyphae seized on the 
moss cells and formed a network of branching 
anastomosing filaments along the whole length of 
the protonema without, however, penetrating the 
cells. If suitable algae were encountered, proper 
thallus formation commenced, and Bonnier con- 
siders that the hyphae receive stimulus and 
nourishment from the protonema sufficient to 
tide them over a considerable period, perhaps until the algal symbiont is 
met. An interesting variation was noted in connection with the cultures of 
Mnium hornum*. If the protonema were of the usual vigorous type, the 
whole length was encased by the hyphal network; but if it were delicate and 
slender, the protoplasm collected in the cell that was touched by hyphae 
and formed a sort of swollen thick-walled bud (Fig. 35). This new body 
persisted when the rest of the filament and the hyphae had disappeared, 
and, in favourable conditions, grew again to form a moss plant. 




g. 34. Germinating hyphae of 
Lecanora subfusca Ach. , grow- 
ing over the alga Vaucheria 
sessilis DC., much magnified 
(after Bonnier). 



F. PARASITISM OF ALGAE ON LICHENS 

A curious instance of undoubted parasitism by an alga, not as in 
Strigula on one of the higher plants, but on a lichen thallus, is recorded 
by Forssell 3 . A group of Protococcns-\ti<& cells established on the thallus 



Bonnier 1888 and 1889*. 



3 Forssell 1884, p. 34. 



66 



CONSTITUENTS OF THE LICHEN THALLUS 



of Peltigera had found their way into the tissue, the underlying cortical 
cells having degenerated. The blue-green cells of the normal gonidial layer 




Fig. 35. Pure culture of protonema of Mnium hornum L. with spores and hyphae of 
Lecidea vernalis Ach. a,a,a, buds forming x 150 (after Bonnier). 

had died off before their advance but no zone was formed by the invading 
algae; they simply withdrew nourishment and gave seemingly no return. 
The phenomenon is somewhat isolated and accidental but illustrates the 
capacity of the alga to absorb food supply from lichen hyphae. 

An instance of epiphytic growth has also been recorded by Zahlbruckner 1 . 
He found an alga, Trentepohlia abietina, covering the thallus of a Brazilian 
lichen, Parmelia isidiophora, and growing so profusely as to obscure the 
isidiose character towards the centre of the thallus. There was no genetic 
connection of the alga with the lichen as the former was not that of the 
lichen gonidium. Lichen thalli are indeed very frequently the habitat of 
green algae, though their occurrence may be and probably is accidental, 

* Zahlbruckner 1902. 



CHAPTER III 

MORPHOLOGY 

GENERAL ACCOUNT OF LICHEN STRUCTURE 
I. ORIGIN OF LICHEN STRUCTURES 

THE two organisms, fungus and alga, that enter into the composition of the 
lichen plant are each characterized by the simplicity of their original structure 
in which there is little or no differentiation into tissues. The gonidia-forming 
algae are many of them unicellular, and increase mainly by division or by 
sporulation into daughter-cells which become rounded off and repeat the life 
of the mother-cell ; others, belonging to different genera, are filaments, 
mostly of single cell-rows, with apical growth. The hyphal elements of the 
lichen are derived from fungi in which the vegetative body is composed of 
branching filaments, a character which persists in the lichen thallus. 

The union of the two symbionts has stimulated both, but more especially 
the fungus, to new developments of vegetative form, in which the fungus, as 
the predominant partner, provides the framework of the lichen plant-body. 
Varied structures have been evolved in order to secure life conditions favour- 
able to both constituents, though more especially to the alga ; and as the 
close association of the assimilating and growing tissues is maintained, the 
thallus thus formed is capable of indefinite increase. 

A. FORMS OF CELL-STRUCTURE 

There is no true parenchyma or cellular structure in the lichen thallus 
such as forms the ground tissue of the higher 
plants. The fungal hyphae are persistently fila- 
mentous and either simple or branched. By 
frequent and regular cell-division always at right 
angles to the long axis and by coherent growth, 
a pseudoparenchyma may however be built up 
which functions either as a protective or strength- 

Lindau 1 proposed the name "plectenchyma" >^^^P S K?S3^3^- 
for the tangled weft of hyphae that is the prin- 

J r Fig. 36. Vertical section of 

cipal tissue system in fungi as well as lichens. young stage of stratose thai- 
The more elaborated pseudoparenchyma he desig- & ^JjSjgSS 
nates as "paraplectenchyma," while the term cortex ; 6, medullary hyphae ; 

, , , i r i /- 1 c< gonidial zone, x 500 (after 

prosoplectenchyma he reserved for the fibrous Schwendener). 

1 Lindau 1899. 

52 




68 MORPHOLOGY 

or chondroid strands of compact filaments that occur frequently in the 
thallus of the larger fruticose lichens, and are of service in strengthening 
the fronds. The term plectenchyma is now generally used for pseudo- 
parenchyma. 

B. TYPES OF THALLUS 

Three factors, according to Reinke 1 , have been of influence in determining 
the thalline development. The first, and most important, is the necessity to 
provide for the work of photosynthesis on the part of the alga. There is 
also the building up of a tissue that should serve as a storage of reserve 
material, essential in a plant the existence of which is prolonged far beyond 
the natural duration of either of the component organisms; and, finally, 
there is the need of protecting the long-lived plant as a whole though more 
particularly the alga. 

Wallroth was the first to make a comparative study of the different 
lichen thalli. He distinguished those lichens in which the green cells and 
the colourless filaments are interspersed equally through the entire thallus 
as "homoiomerous" (Fig. 2), and those in which there are distinct layers of 
cortex, gonidia, and medulla, as "heteromerous" (Fig. i), terms which, 
though now considered of less importance in classification, still persist 
and are of service in describing the position of the alga with regard to the 
general structure. A less evident definition of the different types of thallus 
has been proposed by Zukal 2 who divides them into "endogenous" and 
"exogenous." 

a. ENDOGENOUS THALLUS. The term has been applied to a compara- 
tively small number of homoiomerous lichens in which the alga predominates 
in the development, and determines the form of the thallus. These algae, 
members of the Myxophyceae, are extremely gelatinous, and the hyphae 
grow alongside or within the gelatinous sheath. In the simpler forms the 
vegetative structure is of the most primitive type: the alga retains its 
original character almost unchanged, and the ascomycetous fungus grows 
along with and beside it (Fig. 4). Such are the minutely tufted thalli of 
Thermutis and Spilonema and the longer strands of Epkebe, in which the 
associated Scytonema or Stigonema, filamentous blue-green algae, though 
excited to excessive growth, scarcely lose their normal appearance, making 
it difficult at times to recognize the lichenoid character unless the fruits also 
are present. 

Equally primitive in most cases is the structure of the thallus associated 
with Gloeocapsa. The resulting lichens, Pyrenopsis, Psorotichia, etc. are 
simply gelatinous crusts of the alga with a more or less scanty intermingling 
of fungal hyphae. 

1 Reinke 1895. 2 Zukal l895> p . gfo. 



ORIGIN OF LICHEN STRUCTURES 69 

In the Collemaceae, the gonidial cells of which are species of Nostoc 
(Fig. 2), there appears a more developed thallus; but in general, symbiosis 
in Collema has wrought the minimum of change in the habit of the alga, 
hence the indecision of the earlier botanists as to the identification and 
classification of Nostoc and Collema. Though in many of the species of the 
genus Collema no definite tissue is formed, yet, under the influence of 
symbiosis, the plants become moulded into variously shaped lobes which 
are specifically constant. In some species there is an advance towards 
more elaboration of form in the protective tissues of the apothecia, a layer 
of thin-walled plectenchyma being occasionally formed beneath or around 
the fruit as in Collema granuliferum. 

In all these lichens, it is only the thallus that can be considered as 
primitive: the fruit is a more or less open apothecium more rarely a peri- 
thecium with a fully developed hymenium. Frequently it is provided with 
a protective thalline margin. 

b. EXOGENOUS THALLUS. In this group, composed almost exclusively 
of heteromerous lichens, Zukal includes all those in which the fungus takes 
the lead in thalline development. He counts as such Leptogium, a genus 
closely allied to Collema but with more membranous lobes, in which the 
short terminal cells of the hyphae have united to form a continuous cortex. 
A higher development, therefore, becomes at once apparent, though in some 
genera, as in Coenogonium, the alga still predominates, while the simplest 
forms may be merely a scanty weft of filaments associated with groups of 
algal cells. Such a thallus is characteristic of the Ectolechiaceae, and some 
Gyalectaceae, etc., which have, indeed, been described by Zahlbruckner 1 
as homoiomerous though their gonidia belong to the non-gelatinous 
Chlorophyceae. 

Heteromerous lichens have been arranged by Hue 2 according to their 
general structure in three great series : 

1. Stratosae. Crustaceous, squamulose and foliose lichens with a 
dorsiventral thallus. 

2. Radiatae. Fruticose, shrubby or filamentous lichens with a strap- 
shaped or cylindrical thallus of radiate structure. 

3. Stratosae- Radiatae. Primary dorsiventral thallus, either crustaceous 
or squamulose, with a secondary upright thallus of radiate structure called 
the podetium (Cladoniaceae). 

1 Zahlbruckner 1907. 2 Hue 1899. 



;o MORPHOLOGY 

II. STRATOSE THALLUS 
i. CRUSTACEOUS LICHENS 

A. GENERAL STRUCTURE 

In the series "Stratosae," the plant is dorsiventral, the tissues forming 
the thallus being arranged more or less regularly in strata one above the 
other (Fig. 37). On the upper surface there is a hyphal layer constituting 

I 



Fig. 37. Vertical section of crustaceous lichen (Lecanora subfusca 
var. chlarona Hue) on bark, a, lichen cortex; b, gonidia; 
c, cells of the periderm. x 100. 

a cortex, either rudimentary or highly elaborated ; beneath the cortex is . 
situated the gonidial zone composed of algae and hyphae in close asso- 
ciation ; and deeper down the medulla, generally a loose tissue of branching 
hyphae. The lower cortex which abuts on the medulla may be as fully 
developed as the upper or it may be absent. 

The growing tissue is chiefly marginal ; the hyphae on the outer edge 
remain "meristematic" 1 and provide for horizontal as well as vertical ex- 
tension; and there is also continual increase of the algal cells. There is in 
addition a certain amount of intercalary growth due to the activity of the 
gonidial tissue, both algal and fungal, providing for the renewal of the 
cortex, and even interposing new tissue. 

B. SAXICOLOUS LICHENS 

a. EPILITHIC LICHENS, The crustaceous lichens forming this group 
spread over the rock surfaces. The support must be stable to allow the 
necessary time for the slowly developing organism, and therefore rocks that 
are friable or subject to continual weathering are bare of lichens. 

aa. Hypothallus or Prothallus. The first stage of growth in the lichen 
thallus can be most easily traced in epilithic crustaceous species, especially 
in those that inhabit a smooth rock surface. The spore, on germination, 
produces a delicate branching septate mycelium which radiates on all sides, 
as was so well observed and recorded by Tulasne 2 in Verrucaria muralis 
(Fig. 14). Zukal 3 has called this first beginning the prothallus. In time the 

1 Wainio has adopted this term for growing hyphae 1897, p. 33. 

2 Tulasne 1852. 3 Zukal lg 



STRATOSE THALLUS 



cell-walls of the filaments become much thicker and though, in some species, 
they remain colourless, in others they become dark-coloured, all except the 
extreme tips, owing to the presence of lichen pigments a provision, Zukal 1 
considers, to protect them against the ravages of insects, etc. The pro- 
thallic filaments adhere - closely to the substratum and the branching 
becomes gradually more dendroid in form, though sometimes hyphae are 
united into strands, or even form a kind of plectenchymatous tissue. This 
purely hyphal stage may persist for long 
periods without much change. In time 
there may be a fortuitous encounter with 
the algae (Fig. 38 A) which become the 
gonidia of the plant. Either these have 
been already established on the substra- 
tum as free-growing organisms, or, as 
accidentally conveyed, they alight on the 
prothallus. The contact between alga 
and hypha excites both to active growth 
and to cell-division; and the rapidly 
multiplying gonidia are as speedily sur- 
rounded by the vigorously growing hyphal 
filaments. 

Schwendener 2 has thus described the 
origin and further development of pro- 
thallus and gonidia: on the dark-coloured 
proto- or prothallus, he noted small nestling groups of green cells which 
he, at that time, regarded as direct outgrowths from the lichen hyphae. 
These gonidial cells, increasing by division, multiplied gradually and 
gathered into a connected zone. He also observed that the hyphae in 
contact with the gonidia became more thin-walled and produced many new 
branches. Some of these newly formed branches grow upwards and form 
the cortex, others grow downwards and build up the medulla or pith; the 
filaments at the circumference continue to advance and may start new 
centres of gonidial activity (Fig. 386). In many species, however, this 
prothallus or, as it is usually termed at this stage, the hypothallus, be- 
comes very soon overgrown and obscured by the vigorous increase of the 
first formed symbiotic tissue and can barely be seen as a white or dark line 
bordering the thallus (Fig. 39). Schwendener 3 has stated that probably 
only lichens that develop from the spore are distinguished by a proto- 
thallus, and that those arising from soredia do not form these first creeping 
filaments. 




Fig. 38 A. Hypothallus of Rhizocarpon 
confervoides DC., from the extreme edge, 
with loose gonidia x 600. 



Zukal 1895. 



2 Schwendener 1866. 



3 Schwendener 1863. 



72 MORPHOLOGY 

bb. Formation of crustaceous tissues. Some crustaceous lichens have 
a persistently scanty furfuraceous crust, the vegetative development never 
advancing much beyond the first rather loose association of gonidia and 




Fig. 38 B. Young thallus of Rhizocarpon confervoides DC., with various 
centres of gonidial growth on the hypothallus x 30. 




Fig. 39. Lecanora parella Ach. Determinate thallus with white bordering 
hypothallus, reduced (M. P., Photo.). 



STRATOSE THALLUS 73 

hyphae ; but in those in which a distinct crust or granules are formed, three 
different strata of tissue are discernible: 

1st. An upper cortical tissue of interlaced hyphae with frequent septa- 
tion and with swollen gelatinous walls, closely compacted and with the 
lumen of the cells almost obliterated, not unfrequently a layer of mucilage 
serving as an outer cuticle. This type of cortex has been called by Hue 1 
"decomposed." It is subject to constant surface weathering, thin layers 
being continually peeled off, but it is as continually being renewed endo- 
genously by the upward growth of hyphae from the active gonidial zone. 
Exceptions to this type of cortex in crustaceous lichens are found in some 
Pertusariae where a secondary plectenchymatous cortex is formed, and in 
Dirina where it is fastigiate 2 as in Roccella. 

2nd. The gonidial zone a somewhat irregular layer of algae and 
hyphae below the cortex which varies in thickness according to the species. 

3rd. The medullary tissue of somewhat loosely intermingled branching 
hyphae, with generally rather swollen walls and narrow lumen. It rests 
directly on the substratum and follows every inequality and crack so 
closely, even where it does not penetrate, that the thallus cannot be 
detached without breaking it away. 

In Verrucaria mucosa, a smooth brown maritime lichen found on rocks 
between tide-levels, the thallus is composed of tightly packed vertical rows 
of hyphae, slender, rather thin-walled, and divided into short cells. The 
gonidia are chiefly massed towards the upper surface, but they also occur in 
vertical rows in the medulla. One or two of the upper cells are brown and 
form an even cortex. The same formation occurs in some other sea-washed 
species; the arrangement of the tissue elements recalls that of crustaceous 
Florideae such as Hildenbrandtia, Cruoria, etc. 

cc. Formation of areolae. An "areolate" thallus is seamed and scored 
by cracks of varying width and depth which divide it 
into minute compartments. These cracks or fissures or 
chinks originate in two ways depending on the presence 
or absence of hypothallic hyphae. Where the hypothallus 
is active, new areolae arise when the filaments encounter 
new groups of algae. More vigorous growth starts at once 
and proceeds on all sides from these algal centres, until 

Fig. 40. Young 

similarly formed areolae are met, a more or less pro- thallus of Rhizo- 

nounced fissure marking the limits of each. This primary S^rfc^^'kh 

areolation, termed rimose or rimulose, is well seen in the primary and sub- 
thin smooth thallus of Rhizocarpon geographicum (Fig. 40); 

but the first-formed areolae are also very frequently slightly x 5- 
1 Hue 1906. 2 See p. 83. 




74 MORPHOLOGY 

marked by subsequent cracks due to unequal growth. The areolation caused 
by primary growth conditions tends to become gradually less obvious or to 
disappear altogether. 

Secondary areolation is due to unequal intercalary growth of the 
otherwise continuous thallus 1 . A more active increase of any minute portions 
provokes a tension or straining of the cortex between the swollen areas 
and the surrounding more sluggish tissues ; the surface layers give 
way and chinks arise, a condition described by older lichenologists as 
"rimose-diffract" or sometimes as "rhagadiose." The thallus is generally 
thicker, more broken and granular in the older central parts of the lichen. 
Towards the circumference, where the tissue is thinner and growth more 
equal, the chinks are less evident. Sometimes the more vigorously growing 
areolae may extend over those immediately adjoining, in which case the 
covered portions become brown and their gonidia gradually disappear. 

Strongly marked intersecting lines, similar to those round the margin 
of the thallus, are formed when hypothalli that have themselves started 
from different centres touch each other. A large continuous patch of 
crustaceous thallus may thus be composed of many individuals (Fig. 41). 




Fig. 41. Rhizocarpon geographicum DC. on boulder, reduced (M.P., Photo.}. 

b. ENDOLITHIC LICHENS. In many species, only the lower hyphae 
penetrate the substratum either of rock or soil. In a few, more especially 
those growing on limestone, the greater part or even the whole of the vege- 
tative thallus and sometimes also the fruits are, to some extent, immersed 



1 Malinowski 1911. 



STRATOSE THALLUS 75 

in the rock. It has now been demonstrated that a number of lichens, 
formerly described as athalline, possess a considerable vegetative body 
which cannot be examined until the limestone in which they are embedded 
is dissolved by acids. One such species, Petractis (Gyalecta) exanthematica, 
studied by Steiner 1 and later by Funfstuck 2 , is associated with the blue- 
green filamentous alga, Scytonema, and is homoiomerous in structure, the 
alga growing through and permeating the whole of the embedded thallus. 
A partly homoiomerous thallus, associated with Trentepohlia, has been 
described by Bachmann 3 . He found the bright-yellow filaments of the 
alga covering the surface of a calcareous rock. By reason of their apical 
growth, they pierced the rock and dissolved a way for themselves, not only 
among the loose particles, but right through a clear calcium crystal reaching 
generally to a depth of about 200 /u, though isolated threads had gone 350/1* 
below the surface. Near the outside the tendency was for the algae to 
become stouter and to increase by intercalary growth and by budded yeast- 
like outgrowths; lower down they were somewhat smaller. The hyphae 
that became united with the algae were unusually slender and were charac- 
terized by frequent anastomoses. They closely surrounded the gonidia 
and also filled the loose spaces of the limestone with their fine thread-like 
strands. Though oil was undoubtedly present in the lower hyphae there 
were no swollen nor sphaeroid cells 4 . Some interesting experiments with 
moisture proved that the part of the rock permeated with the lichen 
absorbed much more water and retained it longer than the part that was 
lichen-free. 

Generally the embedded tissues follow the same order as in other 
crustaceous lichens : an upper layer of cortical hyphae, next a gonidial 
zone, and beneath that an interlaced tissue of medullary or rhizoidal hyphae 
which often form fat-cells 4 . Friedrich 5 has given measurements of the 
immersed thallus of Lecanora (Biatorella) simplex: under a cortical layer of 
hyphae there was a gonidial zone 600-700/4 thick, while the lower hyphae 
reached a depth of 1 2 mm. ; he has also recorded an instance of a thallus 
reaching a depth of 30 mm. 

On siliceous rocks such as granite, rhizoidal hyphae penetrate the rock 
chiefly between the thin separable flakes of mica. Bachmann 6 has recog- 
nized in these conditions three distinct series of cell-formations: (i) slender 
long-celled sparsely branched hyphae which form a network by frequent 
anastomoses; (2) further down, though only occasionally, hyphae with 
short thick-walled bead-like cells; and (3) beneath these, but only in or 
near mica crystals, spherical cells containing oil or some albuminous 
substance. 

1 Steiner r88i. 2 Funfstuck 1899. 3 Bachmann 1913. 

4 See p. 215. 5 Friedrich 1906. ' Bachmann 1907. 



;6 MORPHOLOGY 

c. CHEMICAL NATURE OF THE SUBSTRATUM. Lichens growing on 
calcareous rocks or soils are more or less endolithic, those on siliceous 
rocks are largely epilithic, but Bachmann 1 found that the mica crystals in 
granite were penetrated, much in the same way as limestone, by the lichen 
hyphae. These travel through the mica in all directions, though they tend 
to follow the line of cleavage, thus taking the direction of least cohesion. 
He found that oil-hyphae were formed, and also certain peculiar bristle-like 
terminal branches; in other cases there were thin layers of plectenchyma, and 
gonidia were also present. If however felspar or quartz crystals, no matter 
how thin, blocked the way, further growth was arrested, the hyphae being 
unable to pierce through or even to leave any trace on the quartz 2 . On 
granite containing no mica constituents the hyphae can only follow the 
cracks between the different impenetrable crystals. 

Stahlecker 3 has confirmed Bachmann's observations, but he considers 
that the difference in habit and structure between the endolithic and 
epilithic series of lichens is due rather to the chemical than to the physical 
nature of the substratum. Thus in a rock of mixed composition such as 
granite, the more basic constituents are preferred by the hyphae, and are 
the first to be surrounded: mica, when present, is at once penetrated; 
particles of hornblende, which contain 40 to 50 per cent, only of silicic 
acid, are laid hold of by the filaments of the lichen before the felspar, of 
which the acid content is about 60 per cent.; quartz grains which are pure 
silica are attacked last of all. though in the course of time they also become 
corroded. 

The character of the substratum also affects to a great extent the 
comparative development of the different thalline layers: the hyphal tissues 
in silicicolous lichens are much thinner than in lichens on limestone, and 
the gonidial zone is correspondingly wider. In a species of Staurothele on 
granite, Stahlecker 3 estimated the gonidial zone to be about 600/1, thick, 
while the lower medullary hyphae, partly burrowing into the rock, measured 
about 6 mm. Other measurements at different parts of the thallus gave a 
rhizoidal depth of 3 mm., while on a more finely granular substratum, with 
a gonidial zone of 350 p, the rhizoidal hyphae measured only imm. On 
calcareous rocks, on the contrary, with a gonidial zone that is certainly no 
larger, the hyphal elements penetrate the rock to varying depths down to 
1 5 mm. or even more. 

Lang 4 has recorded equally interesting measurements for Sarcogyne 
(Biatorelld) latericola: on slaty rock which contained no mixture of lime, 
the gonidial zone had a thickness of 80 //,, a considerable proportion of the 
very thin thallus. Funfstiick 5 has indeed suggested that this lichen on acid 

1 Bachmann 1904. 2 Bachmann 1904. 3 Stahlecker 1906. 

4 Lang 1903. B Funfstiick 1899. 



STRATQSE THALLUS 77 

rocks is only a starved condition of Sarcogyne (Biatorella) simplex, which on 
calcareous rocks, though with a broader gonidial zone, has, as noted above, 
a correspondingly much larger hyphal tissue. 

Stahlecker's theory is that the hyphae require more energy to grow in 
the acid conditions that prevail in siliceous rocks, and therefore they make 
larger demands on the algal symbionts. It follows that the latter must be 
stimulated to more abundant growth than in circumstances favourable to 
the fungus, such as are found in basic (calcareous) rocks; he concludes that 
on the acid (siliceous) rocks, the epilithic or superficial condition is not only 
a physical but a biological necessity, to enable the algae to grow and 
multiply in a zone well exposed to light with full opportunity for active 
photosynthesis and healthy increase. 

C. CORTICOLOUS LICHENS 

The crustaceous lichens occurring on bark or on dead wood, like those 
on rocks, are either partly or wholly immersed in the substratum (hypo- 
phloeodal), or they grow on the surface (epiphloeodal); but even those with 
a superficial crust are anchored by the lower hyphae which enter any crack 
or crevice of wood or bark and so securely attach the thallus, that it can 
only be removed by cutting away the underlying substance. 

a. EPIPHLOEODAL LICHENS. These lichens originate in the same way 
as the corresponding epilithic series from soredia or from germinating 
spores, and follow the same stages of growth; first a hypothallus with 
subsequent colonization of gonidia, the formation of granules, areolae, etc. 
The small compartments are formed as primary or secondary areolae; the 
larger spaces are marked out by the encounter of hypothalli starting from 
different centres. 

The thickness of the thallus varies considerably according to the species. 
In some Pertusariae with a stoutish irregular crust there is a narrow 
amorphous cortical layer of almost obliterated cells, a thin gonidial zone 
about 35/4 in width and a massive rather dense medulla of colourless 
hyphae. Darbishire 1 has described and figured in Varicellaria microsticta, 
one of the Pertusariaceae, single hyphae that extend like beams across the 
wide medulla and connect the two cortices. In some Lecanorae and Lecideae 
there is, on the contrary, an extremely thin thallus consisting of groups of 
algae and loose fungal filaments, which grow over and between the dead 
cork cells of the outer bark. On palings, there is often a fairly substantial 
granular crust present, with a gonidial zone up to about So/* thick, while 
the underlying or medullary hyphae burrow among the dead wood fibres. 

1 Darbishire 1897. 



78 MORPHOLOGY 

b. HYPOPHLOEODAL LICHENS. These immersed lichens are compar- 
able with the endolithic species of the rock formations, as their thallus is 
almost entirely developed under the outer bark of the tree. They are recog- 
nizable, even in the absence of any fructification, by the somewhat shining 
brownish, white or olive-green patches that indicate the underlying lichen. 
This type of thallus occurs in widely separated families and genera, Lecidea, 
Lecanora, etc., but it is most constant in Graphideae and in those Pyreno- 
lichens of which the algal symbiont belongs to the genus Trentepohlia, 
The development of these lichens is of peculiar interest as it has been 
proved that though both symbionts are embedded in the corky tissues, the 
hyphae arrive there first, and, at some later stage, are followed by the 
gonidia. There is therefore no question of the alga being a "captured 
slave" or "unwilling mate." 

Frank 1 made a thorough study of several subcortical forms. He found 
that \nArthonia radiata, the first outwardly visible indication of the presence 
of the lichen on ash bark was a greenish spot quite distinct from the 
normal dull-grey colour of the periderm. Usually the spots are round in 
outline, but they tend to become ellipsoid in a horizontal direction, being 
influenced by the growth in thickness of the tree. At this early stage only 
hyphae are present; Bornet 2 as well as Frank described the outer periderm 
cells as penetrated and crammed with the colourless slender filaments. 
Lindau 3 , in a more recent work, disputes that statement: he found that the 
hyphae invariably grew between the dead cork cells, splitting them up and 
disintegrating the bark, but never piercing the membranes. The purely 
prothallic condition, as a weft of closely entangled hyphae, may last, Frank 
considers, for a long period in an almost quiescent condition possibly for 
several years before the gonidia arrive. 

It is always difficult to observe the entrance of the gonidia but they 
seem to spread first under the second or third layers of the periderm. With 
care it is possible to trace a filament of Trentepohlia from the surface down- 
wards, and to see that the foremost cell is really the growing and advancing 
apex of the creeping alga. Both symbionts show increased vigour when 
they encounter each other: the thallus at once develops in extent and in 
depth, and, ultimately, reproductive bodies are formed. In some species the 
apothecia or perithecia alone emerge above the bark, in others the outer 
peridermal cells are thrown off, and the thallus thus becomes superficial to 
some extent as a white scurfy or furfuraceous crust. 

The change from a hypophloeodal to a partly epiphloeodal condition 
depends largely on the nature of the bark. Frank 1 found that Lecanora 
pallida remained for a long time immersed when growing on the thick 
rugged bark of oak trunks. When well lighted, or on trees w'ith a thin 

1 Frank 1876. 2 Bornet 1873, p. 81. 3 Lindau 1895. 



STRATOSE^THALLUS 79 

periderm, such as the ash, the lichen emerges much earlier and becomes 
superficial. 

Black (or occasionally white) lines intersect the thallus and mark, as in 
saxicolous lichens (Fig. 41), the boundary lines between different indi- 
viduals or different species. The pioneer hyphae of certain lichens very 
frequently become dark-coloured, and Bitter 1 has suggested as the reason 
for this that in damp weather the hypothallic growth is exceptionally 
vigorous. When dry weather supervenes, with high winds or strong sun- 
shine, the outlying hyphae, unprotected by the thallus, become dark- 
coloured. On the return of more normal conditions the blackened tips are 
thrown off. Bitter further states that species of Graphideae do not form a 
permanent black limiting line when they grow in an isolated position: it is 
only when their advance is checked by some other thallus that the dark per- 
sistent edge appears, a characteristic also to be seen in the crust of other 
lichens. The dark boundary is always more marked in sunny exposed 
situations: in the shade, the line is reduced to a mere thread. 

Bitter's restriction of black boundary lines to cases of encountering 
thalli only, would exclude the comparison one is tempted to make between 
the advancing hyphae of lichens and those of many woody fungi where the 
extreme edge of the white invaded woody tissue is marked by a dark line. 
In the latter case however it is the cells of the host that are stained black 
by the fungus pigment. 

2. SQUAMULOSE LICHENS 

A. DEVELOPMENT OF THE SQUAMULE 

The crustaceous thallus is more or less firmly adherent to, or confused 
with, the substratum. Further advance to a new type of thallus is made 
when certain hyphal cells of soredium or granule take the lead in an 
ascending direction both upwards and outwards. As growth becomes 
definitely apical or one-sided, the structure rises free from the substratum, 
and small lobules or leaflet-like squamules are formed. Each squamule 
in this type of thallus is distinct in origin and not merely the branch of 
a larger whole. 

In a few lichens the advance from the crustaceous to the squamulose 
structure is very slight. The granules seem but to have been flattened out 
at one side, and raised into minute rounded projections such as those that 
compose the thallus of Lecanora badia generally described as "subsquamu- 
lose." The squamulose formation is more pronounced in Lecidea ostreata, 
and in some species of Pannaria ; and the whole thallus may finally consist 
of small separate lobes as in Lecidea lurida, Lecanora crassa, L. saxicola, 

1 Bitter 1899. 



8o 



MORPHOLOGY 



species of Dermatocarpon and the primary thallus of the Cladoniae. Most of 
these squamules are of a firm texture and more or less round in outline; in 
some species of Cladonia, etc., they are variously crenate, or cut into pinnate- 
like leaflets. Squamulose lichens grow mostly on rocks or soil, occasionally on 
dead wood, and are generally attached by single rhizoidal hyphae, either 
produced at all points of the under surface, or from the base only, growth 
in the latter case being one-sided. In a few instances, as in Heppia Guepint, 
there is a central hold-fast. 

A frequent type of squamulose thallus is that termed "placodioid," or 
"effigurate," in which the squamulose character is chiefly apparent at the 
circumference. The thallus is more or less orbicular in 
outline ; the centre may be squamulose or granular and 
cracked into areolae ; the outer edge is composed of 
radiating lobules closely appressed to the substratum 
(Fig. 42). 

All lichens with this type of thallus were at one time 
included in the genus Placodium, now restricted by some 
lichenologists to squamulose or crustaceous species with 
polarilocular spores. Many of them rival Xanthoria parietina in their 
brilliant yellow colouring. 




Fig. 42. Placodium 
murorum DC. 
Part of placodioid 
thallus with apo- 
thecia x i. 




Fl 'g- 43- Lecania candicans A. Zahlbr., with placodioid thallus, 
reduced (S. H., Photo.}. 

There are also greyish-white effigurate lichens such as Lecanora saxicola, 
Lecania candicans (Fig. 43) and Buellia canescens, well-known British 
species. 



STRATOSE THALLUS 81 



B. TISSUES OF SQUAMULOSE THALLUS 

The anatomical structure of the squamules is in general somewhat 
similar to that of the crustaceous thallus: an upper cortex, a gonidial zone, 
and below that a medullary layer of loose hyphae with sometimes a lower 
cortex. 

1. The upper cortex, as in crustaceous lichens, is generally of the 
"decomposed" 1 or amorphous type: interlaced hyphae with thick gelatinous 
walls. A more highly developed form is apparent in Parmeliella and 
Pannaria where the upper cortex is formed of plectenchyma, while in the 
squamules of Heppia the whole structure is built up of plectenchyma, with 
the exception of a narrow band of loose hyphae in the central pith. 

2. The gonidia are Myxophyceae or Chlorophyceae ; the squamules in 
some instances may be homoiomerous as in Lepidocollema, but generally 
they belong to the heteromerous series, with the gonidia in a circumscribed 
zone, and either continuous or in groups. Friedrich 2 held that, as in crus- 
taceous lichens the development of the gonidial as compared with the other 
tissues depended on the substratum. The squamules of Pannaria micro- 
phylla on sandstone were lOOyu- thick, and the gonidial layer occupied 80 or 
90 /A of the whole 3 . With that may be compared Placodium Garovagli on 
lime-containing rock: the gonidial layer measured only 50 //. across, the 
pith hyphae 280 p and the rhizoidal hyphae that penetrated the rock 500 //.. 

3. The medullary layer, as a rule, is of closely compacted hyphae which 
give solidity to the squamules; in those of Heppia it is almost entirely 
formed of plectenchyma. 

4. The lower cortex is frequently little developed or absent, especially 
when the squamules are closely applied to the support as in some species 
of Dennatocarpon. In some of the squamulose Lecanorae (L. crassa and 
L. saxicoUi) the lowest hyphae are somewhat more closely interwoven; 
they become brown in colour, and the lichen is attached to the substratum 
by rhizoid-like branches. In Lecanora lentigera there is a layer of parallel 
hyphae along the under surface. Further development is reached when 
a plectenchyma of thick-walled cells is formed both above and below, as in 
Psoroma hypnorum, though on the under surface the continuity is often 
broken. The squamules of Cladoniae are described under the radiate-stratose 
series. 

1 See p. 83. - Friedrich 1906. 3 See p. 76. 



82 MORPHOLOGY 

3. FOLIOSE LICHENS 

A. DEVELOPMENT OF FOLIOSE THALLUS 

The larger leafy lichens are occasionally monophyllous and attached at 
a central point as in Umbilicaria, but mostly they are broken up into lobes 
which are either imbricate and crowded, or represent the dividing and 
branching of the expanding thallus at the circumference. They are hori- 
zontal spreading structures, with marginal and apical growth. The several 
tissues of the squamule are repeated in the foliose thallus, but further pro- 
vision is made to meet the requirements of the larger organism. There is the 
greater development of cortical tissue, especially on the lower surface, and 
the more abundant formation of rhizoidal organs to attach the large flat 
fronds to the support. There are also various adaptations to secure the aera- 
tion of the internal tissues 1 . 

B. CORTICAL TISSUES 

Schwendener 2 was the first who, with the improved microscope, made 
a systematic study of the minute structure of lichens. He examined typical 
species in genera of widely different groups and described their anatomy in 
detail. The most variable and perhaps the most important of the tissues 
of lichens is the cortex, which is most fully developed in the larger thalli, and 
as the same type of cortical structures recurs in lichens widely different in 
affinity as well as in form, it seems well to group together here the ascertained 
facts about these covering layers. 

a. TYPES OF CORTICAL STRUCTURE. Zukal 3 , and more recently 
Hue 4 , have made independent studies in the comparative morphology of 
the thallus and have given particular attention to the different varieties 
of cortex. They each find that the variations come under a definite series 
of types. Zukal recognized five of these : 

1. Pseudoparenchymatous (plectenchyma): by frequent septation of 
regularly arranged hyphae and by coalescence a kind of continuous cell- 
structure is formed. 

2. Palisade cells: the outer elongate ends of the hyphae lie close 
together in a direction at right angles to the surface of the thallus and form 
a coherent row of parallel cells. 

3. Fibrous: the cortical hyphae lie in strands of fine filaments parallel 
with the surface of the thallus. 

4. Intricate: hyphae confusedly interwoven and becoming dark in 
colour form the lower cortex of some foliose lichens. 

1 See p. 126. 2 Schwendener 1860, 1863 and 1868. 3 Zukal 1895, p. 1305. 4 Hue 1906. 



STRATOSE THALLUS 



These four types, Zukal finds, are practically without interstices in the 
:issue and form a perfect protection against excessive transpiration. He adds 
^et another form: 

5. A cortex formed of hyphae with dark-coloured swollen cells, 
,vhich is not a protection against transpiration. It occurs among lower crus- 
:aceous forms. 

Hue has summed up the different varieties under four types, but as he 
las omitted the "fibrous" cortex, we arrive again at five different kinds of 
:ortical formation, though they do not exactly correspond to those of 
ukal. A definite name is given to each type: 

i. Intricate : an intricate dense layer of gelatinous-walled hyphae, 
Dranching in all directions, but not coalescent (Fig. 44). This rather unusual 
:ype of cortex occurs in Sphaerophorus and Stereocanlon, both of which 
lave an upright rigid thallus (fruticose). 
f 





Fig. 44. Sphaerophorus coralloides Pers. Trans- 
verse section of cortex and gonidial layer 
near the growing point of a frond x 600. 



Fig. 45. Roccella fudformis'DC. Trans- 
verse section of cortex near the 
growing point of a frond x 600. 



2. Fastigiate : the hyphae bend outwards or upwards to form the 
:ortex. A primary filament can be distinguished with abundant branches, 
all tending in the same direction; anastomosis may take place between the 
hyphae. The end branches are densely packed, though there are occasional 
interstices (Fig. 45). Such a cortex occurs in Thamnolia\ in several genera 
af Roccellaceae Roccellographa^ Roccellina, Reinkella, Pentagenella, Combea, 
Schizopelte and Roccella and also in the crustaceous genus Dirina. The 
fastigiate cortex corresponds with Zukal's palisade cells. 

3. Decomposed: in this, the most frequent type of cortex, the hyphae 
that travel up from the gonidial layer become irregularly branched and 
frequently septate. The cell-walls of the terminal branches become swollen 
into a gelatinous mass, the transformation being brought about by a change 

62 



8 4 



MORPHOLOGY 




in the molecular constituents of the cell-walls which permits the imbibitior 
and storage of water. The tissue, owing to the enormous increase of the 
wall, is so closely pressed together that the individua 
hyphae become indistinct; the cell-lumen finall} 
disappears altogether, or, at most, is only to be 
detected in section as a narrow disconnected dart 
streak. The decomposed cortex is characteristic 
of many lichens, crustaceous (Fig. 46) and squamu- 
lose, as well as of such highly developed genera as 
Usnea, Letharia, Ramalina, Cetraria, Evernia anc 
certain Parmeliae. 

Zukal took no note of the decomposed cortex 
but the omission is intentional and is due to his 
regarding the structure of the youngest stages of the 
thallus near the growing point as the most typical and as giving the besi 
indication as to the true arrangement of hyphae in the cortex. He thu5 
describes palisade tissue as the characteristic cortex of Evernia, since the 
formation near the growing point of the fronds is somewhat palisade-like 
and he finds fibrous cortex at the tips of Usnea filaments. In both these 
instances Hue has described the cortex as decomposed because he takes 
account only of the fully formed thallus in which the tissues have reached 
a permanent condition. 

4. Plectenchymatous: the last of Hue's types corresponds with the 
first described by Zukal.^It is the result of the lateral coherence and frequent 
septation of the hyphae into short almost square or rounded cells (Fig. 47) 
The simplest type of such a cortex can be studied in Leptogium t a genus oi 



Fig. 46. Lecanora glaucoma 
va.r.corrugata'Ny\. Vertical 
section of cortex x 500 (after 
Hue). 




Fig. 47. Peltigera canina DC. Vertical section 
of cortex and gonidial zone x 600. 



STRATOSE THALLUS 85 

gelatinous lichens in which the tips of the hyphae are cut off at the surface 
by one or more septa. The resulting cells are wider than the hyphae and 
they cohere together to form, in some species, disconnected patches of cells; 
in others, a continuous cortical covering one or more cells thick, while in 
the margin of the apothecium they form a deep cellular layer. The cellular 
type of cortex is found also, as already stated, in some crustaceous Pertu- 
sariae, and in a few squamulose genera or species. It forms the uppermost 
layer of the Peltigera thallus and both cortices of many of the larger foliose 
lichens such as Sticta, Parmelia, etc. 

5. The "fibrous" cortex must be added to this series, as was pointed 
out by Heber Howe 1 who gave the less appropriate designation of "simple" 
to the type. It consists of long rather sparingly branched slender hyphae 
that grow in a direction parallel with the surface of the thallus (Fig. 48). 
It is characteristic of several fruticose and foliose lichens with more or less 
upright growth, such as we find in several of the Physciae, and in the allied 
genus TeloschisteS) in Alectoria, several genera of Roccellaceae, in Usnea 
longissima and in Parmelia pubescens, etc. Zukal would have included all 
the Usneae as the tips are fibrous. 




Fig. 48. Physcia ciliaris DC. Vertical section of thallus. a, cortex; 
b, gonidial zone; c, medulla, x 100. 

More than one type of cortex, as already stated, may appear in a genus; 
a striking instance of variability occurs in Solorina where, as Hue 2 has 
pointed out, the cortex of .S". octospora is fastigiate, that of all the other 
species being plectenchymatous. Cortical development is a specific rather 
than a generic characteristic. 

b. ORIGIN OF VARIATION IN CORTICAL STRUCTURE. The immediate 
causes making for differentiation in cortical development are: the prevailing 
direction of growth of the hyphae as they rise from the gonidial zone; the 
amount of branching and the crowding of the filaments ; the frequency of 
septation ; and the thickening or degeneration of the cell-walls which may 

1 Heber Howe 1912. Hue 1911. 



86 MORPHOLOGY 

become almost or entirely mucilaginous. In the plectenchymatous cortex, 
the walls may remain quite thin and the cells small as in Xanthoria parie- 
tina, or the walls may be much thickened as in both cortices of Sticta. 
As a result of stretching the cell may increase enormously in size: in some 
instances where the internal hyphae are about 3 ft to 4 /A in width, the 
cortical cells formed from these hyphae may have a cell cavity 15 /j, to 16/1, 
in diameter. 

c. Loss AND RENEWAL OF CORTEX. Very frequently the cortex is 
covered over by a layer of homogeneous mucilage which forms an outer 
cuticle. It arises from the continual degeneration of the outer cell-walls 
and it is liable to friction and removal by atmospheric agency as was 
first described by Schwendener 1 in the weather-beaten cortex of Umbi- 
licaria pustulata. He had noted the irregular jagged outline of the cross 
section of the thallus, and he then suggested, as the probable reason, the 
decay of the outer rind with the constant renewal of it by the hyphae from 
the underlying gonidial zone, though he was unable definitely to prove his 
theory. The peeling of the dead outer layer (with its replacement by new 
tissue) has however been observed many times since his day. It has been 
described by Darbishire 2 in Pertusaria: in that genus there is at first a 
primary cortex formed of hyphae that grow in a radial direction, parallel 
to the surface of the thallus. The walls of these hyphae become gradually 
more and jmore mucilaginous till the cells are obliterated. Meanwhile 
short-celled filaments grow up in serried ranks from the gonidial layer and 
finally push off the dead "fibrous" cortex. The new tissue takes on a 
plectenchymatous character, and the outer cells in time become decomposed 
and provide a mucilaginous cuticle which in turn is also subject to wasting. 

The same process of peeling was noted by Rosendahl 3 in some species of 
brown Parmeliae, where the dead tissues were thrown off in shreds, though 
only in isolated patches. But whether in patches or as a continuous sheath, 
there is constant degeneration, with continual renewal of the dead material 
from the internal tissues. 

The cortex is the most highly developed of all the lichen structures and 
is of immense importance to the plant as may be judged from the various 
adaptations to different needs 4 . The cortical cell-walls are frequently 
impregnated with some dark-coloured substance which, in exposed situa- 
tions, must counteract the influence of too direct sunlight and be of 
service in sheltering the gonidia. Lichen acids sometimes very brightly 
coloured and oxalic acid are deposited in the cortical tissues in great 
abundance and aid in retaining moisture; but the two chief functions to 

1 Schwendener 1863, p. 180. a Darbishire 1897. * R ose ndahl 1907. 

4 See p. 96. 



STRATOSE THALLUS 87 

which the cortex is specially adapted are the checking of transpiration and 
the strengthening of the thallus against external strains. 

d. CORTICAL HAIRS OR TRICHOMES. Though somewhat rare, cortical 
hairs are present on the upper surface of several foliose lichens. They take 
rise, in all the instances noted, as a prolongation of one of the cell-rows 
forming a plectenchymatous cortex. 

In Peltidea (Peltigerd) apJithosa they are especially evident near the 
growing edges of the thallus; and they take part in the development of 
the superficial cephalodia 1 which are a constant feature of the lichen. They 
tend to disappear with age and leave the central older parts of the thallus 
smooth and shining. In several other species of Peltigera (P. canina, etc.) 
they are present and persist during the life of the cortex. In these lichens 
the cells of the cortical tissue are thin-walled, all except the outer layer, 
the membranes of which are much thicker. The hairs rising from them are 
also thick-walled and septate. Generally they branch in all directions and 
anastomose with neighbouring hairs so that a confused felted tangle is 
formed; they vary in size but are, as a rule, about double the width of the 
medullary hyphae as are the cortical cells from which they rise. They disap- 
pear from the thallus, frequently in patches, probably by weathering, but 
over large surfaces, and especially where any inequality affords a shelter, 
they persist as a soft down. 

Hairs are also present on the upper surface of some Parmeliae. Rosen- 
dahl 2 has described and figured them in P. glabra and P. verniculifera 
short pointed unbranched hyphae, two or more septate and with thickened 
walls. They are most easily seen near the edge of the thallus, though they 
persist more or less over the surface; they also grow on the margins of the 
apothecia. In P. verruculifera they arise from the soredia; in P. glabra 
a few isolated hairs are present on the under surface. 

In Nephromium tomentosum there is a scanty formation of hairs on the 
upper surface. They are abundant on the lower surface, and function as 
attaching organs. A thick tomentum of hairs is similarly present on the 
lower surface of many of the Stictaceae either as an almost unbroken 
covering or in scattered patches. In several species of Leptoginm they grow 
out from the lower cortical cells and attach the thin horizontal fronds ; and 
very occasionally they are present in Collema. 

C. GONIDIAL TISSUES 

With the exception of some species of Collema and Leptogium lichens 
included under the term foliose, are heteromerous in structure, and the algae 
that form the gonidial zone are situated below the upper cortex and, there- 

1 See p. 133. 2 Rosendahl 1907. 



88 MORPHOLOGY 

fore, in the most favourable position for photosynthesis. Whether belonging 
to the Myxophyceae or the Chlorophyceae, they form a green band, straight 
and continuous in some forms, in others somewhat broken up into groups. 
In certain species they push up at intervals among the cortical cells, as in 
Gyropkora and in Parmelia tristis. In Solorina crocea a regular series of 
gonidial pyramids rises towards the upper surface. The green cells are 
frequently more dense at some points than at others, and they may pene- 
trate in groups well into the medulla. 

The fungal tissue of the gonidial zone is composed of hyphae which 
have thinner walls, and are generally somewhat loosely interlacing. In 
Peltigera^ the gonidial hyphae are so connected by frequent branching and 
by anastomosis that a net-like structure is formed, in the meshes of which 
the algae a species of Nostoc are massed more or less in groups. In 
lichens with a plectenchymatous cortex, the cellular tissue may extend 
downwards into the gonidial zone and the gonidia thus become enmeshed 
among the cells, a type of formation well seen in the squamulose species, 
Dermatocarpon lachneum and Heppia Guepini, where the massive plecten- 
chyma of both the upper and lower cortices encroaches on the pith. In 
Endocarpon and in Psoroma the gonidia are also surrounded by short cells. 

A similar type of structure occurs in Cora Pavonia, one of the Hymeno- 
lichenes: the gonidial hyphae in that species form a cellular tissue in which 
are embedded the blue-green Chroococcus cells 2 . 

D. MEDULLA AND LOWER CORTEX 

a. MEDULLA. The hyphal tissue of the dorsi ventral thallus that lies 
between the gonidial zone and the lower cortex or base of the plant is 
always referred to as the medulla or pith. It is, as a rule, by far the most 
considerable portion of the thallus. In Parmelia caperata (Fig. 49), for 
instance, the lobes of which are about 300 /it thick, over 200 p of the space 
is occupied by this layer. It varies however very largely in extent in 
different lichens according to species, and also according to the substratttm. 
In another Parmelia with a very thin thallus, P. alpicola growing on quart- 
zite, the medulla measures scarcely twice the width of the gonidial zone. 

It forms a fairly massive tissue in some of the crustaceous lichens in some 

Pertusariae and Lecanorae attaining a width of about 600 /*. 

Nylander 3 distinguished three types of medullary tissue in lichens: 

(1) felted, which includes all those of a purely filamentous structure; 

(2) cretaceous or tartareous, more compact than the felted, and containing 
granular or crystalline substances as in some Pertusariae; and lastly 

(3) the cellular medulla in which the closely packed hyphae are divided 

1 Me y er '9 2 - 2 See p. 52. 3 Nyland e r l8s8 . 



STRATOSE THALLUS 



89 



into short cells and a kind of plectenchyma is formed, as in Lecanora 
(Psoromd) hypnorum, in Endocarpon, etc. 




Fig. 49. Parmelia caperala Ach. (S. H., Photo.}. 

The felted medulla is characteristic of most lichens and is formed of 
loose slender branching septate hyphae with thickish walls. This interwoven 
hyphal texture provides abundant air-spaces. 

Hue 1 has noted that the walls of the medullary hyphae in Parmeliae are 
smooth, unless they have been exposed to great extremes of heat or cold, 
when they become wrinkled or scaly. They are very thick-walled in Pelti- 
gera (Fig. 50). 




Fig. 50. Hyphae .from lower medulla of Peltigera canina DC. x 600. 
1 Hue 1898. 



MORPHOLOGY 



b. LOWER CORTEX. In some foliose lichens such as Peltigera there is 
no special tissue developed on the under surface. In Lobaria pulmonaria 
large patches of the under surface are bare, and the medulla is exposed to 
the outer atmosphere, sheltered only by its position. In some other lichens 
the lowermost hyphae lie closer together and a kind of felt of almost parallel 
filaments is formed, generally darker in colour, as in Lecanora lentigera, and 
in some species of Physcia. 

Most frequently however the tissues of the upper cortex are repeated on 
the lower surface, though differing somewhat in detail. In all of the brown 
Parmeliae, according to Rosendahl 1 , the structure is identical for both 
cortices, though the upper develops now hairs, now isidia, breathing pores, 
etc., while the lower produces rhizinae. The amorphous mucilaginous cuticle 
so often present on the upper surface is absent from the lower, the walls 
of the latter being often charged instead with dark-brown pigments. 

c. HYPOTHALLIC STRUCTURES. An unusual development of hyphae 
from the lower cortex occurs i-n the genera Anzia and Pannoparmelia both 

closely related to Parmelia whereby a 
loose sponge-like hypothallus of anasto- 
mosing reticulate strands is formed. In 
one of the simpler types, Anzia colpodes, 
a North American species, the hyphae 
passing out from the lower medulla be- 
come abruptly dark-brown in colour, and 
are divided into short thick-walled cells. 
Frequent branching and anastomosis of 
these hyphae result in the formation of 
a cushion-like structure about twice the 
bulk of the thallus. In another species 
from Australia (A. Japonica) there is a 
lower cortex, distinct from the medulla, 
consisting of septate colourless hyphae 
with thick walls. From these branch out 
free fi laments, similar in structure but dark 
in colour, which branch and anastomose 
as in the previous species. 

In Pannoparmelia the lower cortex 
and the outgrowths from it are several 
cells thick; they may be thick- walled as 
in Anzia, or they may be thin-walled as 
described and figured by Darbishire 2 in 




Fig. 51. Pannoparmelia anzioides Darb. 
Vertical section of thallus and hypo- 
thallus. 0, cortex ; b, gonidial zone ; 
i, medulla; d, lower cortex; e, hypo- 
thallus. x ca. 450 (after Darbishire). 



Rosendahl 1907. 



2 Darbishire 1912. 



STRATOSE THALLUS 91 

P annoparmelia anzioides, a species from Tierra del Fuego (Fig. 51). A some- 
what dense interwoven felt of hyphae occurs also in certain parts of the 
under surface of Parmelia physodes*. 

This peculiar structure, regarded as a hypothallus, is probably of service 
in the retention of moisture. The thick cell-walls in most of the forms 
suggest some such function. 

E. STRUCTURES FOR PROTECTION AND ATTACHMENT 

Such structures are almost wholly confined to the larger foliose and 
fruticose lichens and are all of the same simple type ; they are fungal 
in origin and very rarely are gonidia associated with them. 

a. CILIA. In a few widely separated lichens stoutish cilia are borne, 
mostly on the margins of the thallus lobes, or on the margins of the apo- 




Fig. 52. Usneaflorida Web. Ciliate apothecia (S. H., Pkoto.). 

thecia (Fig. 52). They arise from the cortical cells or hyphae, several of 
which grow out in a compact strand which tapers gradually to a point. 
Cilia vary in length up to about I cm. or even longer. In some lichens they 

1 Porter 1919. 



9 2 



MORPHOLOGY 



retain the colour of the cortex and are greyish or whitish-grey, as in Physcia 
ciliaris or in Physcia hispida (Fig. 1 10). They provide a yellow fringe to 
the apothecia of Physcia chrysophthalma and a green fringe to those of 
Usnea fiorida. They are dark-brown or almost black in Parmelia perlata 
var. ciliata and in P. cetrata, etc. as also in Gyrophora cylindrica. The fronds 
of Cetraria islandica and other species of the genus are bordered with short 
spinulose brown hairs whose main function seems to be the bearing of 
"pycnidia" though in many cases they are barren (Fig. 128). 

Superficial cilia are more rarely formed than marginal ones, but they are 
characteristic of one not uncommon British species, Parmelia proboscidea 
(P. pilosella Hue). Scattered over the surface of that lichen are numerous 
crowded groups of isidia which, frequently, are prolonged upwards as dark- 
brown or blackish cilia. Nearly every isidium bears a small brown spot on 
the apex at an early stage of growth. Similar cilia are sparsely scattered 
over the thallus, but their base is always a rather stouter grey structure, 
which suggests an isidial origin. Cilia also occur on the margin of the lobes. 

As lichens are a favourite food of snails, insects, etc., it is considered 
that these structures are protective in function, and that they impede, if 
they do not entirely prevent, the larger marauders in their work of destruction. 

b. RHIZINAE. Lichen rootlets are mainly for the purpose of attachment 
and have little significance as organs of absorption. They have been noted 
in only one crustaceous lichen, Varicellaria microsticta 1 , an alpine species 
that spreads over bark or soil, and which is further distinguished by being 





Fig. 53. Rhizoid of Parmelia exasferata Carroll (P. aspidota Rosend.). A, hyphae growing out 
from lower cortex x 450. B, tip of rhizoid with gelatinous sheath x 335 (after Rosendahl). 

provided with a lower cortex of plectenchyma. In foliose lichens they are 
frequently abundant, though by no means universal, and attach the spreading 
fronds to the support. They originate, as Schwendener 2 pointed out, from 
the outer cortical cells, exactly as do the cilia, and are scattered over the 
1 Darbishire 1897. 2 Schwendener 1860. 



STRATOSE THALLUS 



93 



under surface or are confined to special areas. Rosendahl 1 has described 
their development in the brown species of Parmeliae: the under cortex in 
these lichens is formed of a cellular plectenchyma with thickish walls ; the 
rootlets arise by the outgrowth of several neighbouring cells from some slight 
elevation near the edge of the thallus. Branching and interlacing of these 
growing rhizinal hyphae follow, the outermost frequently spreading outwards 
at right angles to the axis, and forming a cellular cortex. The apex of the 
rhizoid is generally an enlarged tuft of loose hyphae involved in mucilage 
(Fig- 53), a provision for securing firmer cohesion to the support; or the 
tips spread out as a kind of sucker. Not unfrequently neighbouring "rootlets" 
are connected by mucilage at the tips, or by outgrowths of their hyphae, 
and a rather large hold-fast sheath is formed. 

In species of Peltigera (Fig. 54) the rhizinae are confined to the veins 
or ridges (Fig. 55); they are thickish at the base, and are generally rather 





Fig. 54. Peltigera canina DC. (S. H., Photo ). 



r ig- 55- 

Under surface with veins and 
rhizoids (after Reinke). 



long and straggling. Meyer 2 states that the central hyphae are stoutish 
and much entangled owing to the branching and frequent anastomosis of 
one hypha with another; the peripheral terminal branches are thinner-walled 
and free. These rhizinae vary in colour from white in Peltigera canina to 
brown or black in other species. Most species of Peltigera spread over grass 
or mosses, to which they cling by these long loose "rootlets." 

Lichen rhizinae, distinguished by Reinke 3 as "aerial rhizinae," are more 



1 Rosendahl 1507. 



2 Meyer 1902. 



3 Reinke 1895, p. 186. 



94 MORPHOLOGY 

or less characteristic of all the species of Parmelia with the exception of 
those belonging to the subgenus Hypogymnia in which they are of very rare 
occurrence, arising, according to Bitter 1 , only in response to some external 
friction. They are invariably dark-coloured, rather short, about one to a 
few millimetres in length, and are simple or branched. The branches may 
go off at any angle and are sometimes curved back at the ends in anchor- 
like fashion. The Parmeliae grow on firm substances, trees, rocks, etc., and 
the irregularities of their attaching structures are conditioned by the obstacles 
encountered on the substratum. Not unfrequently the lobes are attached 
by the rhizinae to underlying portions of the thallus. 

In the genus Gyrophora, the rhizinae are simple strands of hyphae 
(G. polyrhizd) or they are corticate structures (G. murina, G. spodochroa 
and G. vellea\ They are also present in species of Solorina, Ricasolia, 
Sticta and Physcia and very sparingly in Cetraria (Platysma). 

c. HAPTERA. Sernander 2 has grouped all the more distinctively aerial 
organs of attachment, apart from rhizinae, under the term "hapteron" and he 
has described a number of instances in which cilia and even the growing 
points of the thallus may become transformed to haptera or sucker-like 
sheaths. 

The long cilia of Physcia ciliaris occasionally form haptera at their tips 
where the hyphae are loose and in active growing condition. Contact with 
some substance induces branching by which a spreading sheath arises; a 
plug-like process may also be developed which pierces the substance en- 
countered not unfrequently another lobe of its own thallus. The long 
flaccid fronds of Evernia furfuracea are frequently connected together by 
bridge-like haptera which rise at any angle of the thallus or from any part 
of the surface. 

The spinous hairs that border the thalline margins in Cetraria may also, 
in contact with some body often another frond of the lichen form a 
hapteron, either while the spermogonium, which occupies the tip of the 
spine, is still in a rudimentary stage, or after it has discharged its spermatia. 
The small sucker sheath may in that case arise either from the apex of the 
cilium, from the wall of the spermogonium or from its base. By means of 
these haptera, not only different individuals become united together, but 
instances are given by Sernander in which Cetraria islandica, normally a 
ground lichen, had become epiphytic by attaching itself in this way to the 
trunk of a tree (Pinus sylvestris}. 

In Alectoria, haptera are formed at the tip of the thallus filament as an 
apical cone-like growth from which hyphae may branch out and penetrate 
any convenient object. A species of this genus was thus found clinging to 

1 Bitter 1901. 2 Sernander 1901. 



STRATOSE THALLUS 95 

stems of Betula nana. Apical haptera are very frequent in Cladonia rangi- 
ferina and Cl. sylvatica, induced here also by contact. These two plants, as 
well as several species of Cetraria, tend, indeed, to become entirely epiphytic 
on the heaths of the Calluna formations. Haptera similar to those of Alectoria 
occur in Usnea, Evernia, Ramalina and Cornicularia (Cetraria). In Evernia 
prunastri var. stictoceros, a heath form, the fronds become attached to the 
stems and branches of Erica tetralix by hapteroid strands of slender glutinous 
hyphae which persist on the frond of the lichen after it is detached as 
small very dark tubercles surmounted, as Parfitt 1 pointed out, by a dark- 
brown grumous mass of cells. Plug-like haptera may be formed at the base 
of Cladoniae which attach them to each other and to the substratum. The 
brightly coloured fronds of Letharia vulpina are attached to each other in 
somewhat tangled fashion by lateral bridges or by fascicles of hyphae dark- 
brown at the base but colourless at the apices, exactly like aerial adventitious 
rhizinae. They grow out from the fronds generally at or near the tips and 
lay hold of a neighbouring frond by means of mucilage. These haptera are 
evidently formed in response to friction. Haptera along with other lichen 
attachments have received considerable attention from Gallic 2 . He finds 
them arising on various positions of the lichen fronds and has classified 
them accordingly. 

After the haptera have become attached, they increase in size and strength 
and supply a strong anchorage for the plant; the point of contact frequently 
forms a basis for renewed growth while the part beneath the hapteron may 
gradually die off. Haptera are more especially characteristic of fruticose 
lichens, but Sernander considers that the rhizinae of foliose species may 
function as haptera. They are important organs of tundra and heath 
formations as they enable the lichens to get a foothold in well-lighted 
positions, and by their aid the fronds are more able to resist the extreme 
tearing strains to which they are subjected in high and unsheltered moor- 
lands. 

F. STRENGTHENING TISSUES OF STRATOSE LICHENS 

Squamulose and foliose lichens grow mostly in close relation with the 
support, and the flat expanding thallus, as in the Parmeliae, is attached at 
many points to the substance tree, rock, etc. over which the plants spread. 
Special provision for support is therefore not required, and the lobes remain 
thin and flaccid. Yet, in a number of widely different genera the attachment 
to the substratum is very slight, and in these we find an adaptation of 
existing tissues fitted to resist tearing strains, resistance being almost 
invariably secured by the strengthening of the cortical layers. 

1 Parfitt in Leighton 1871, p. 470. 2 Gallic 1915. 



96 MORPHOLOGY 

a. BY DEVELOPMENT OF THE CORTEX. Such a transformation of tissue 
is well illustrated in Heppia Guepini. The thallus consists of rigid squamules 
which are attached at one point only ; the cortex of both surfaces is plecten- 
chymatous and very thick and even the medulla is largely cellular. 

The much larger but equally rigid coriaceous thallus of Dermatocarpon 
miniatum (Fig. 56) has also a single central attachment or umbilicus, and 




Fig- 56. Dermatocarpon miniatum Th. Fr. (S. H., Photo.). 

both cortices consist of a compact many-layered plectenchyma. The same 
structure occurs in Umbilicaria pitstnlata and in some species of Gyrophora, 
which, having only a single central hold-fast, gain the necessary stiffening 
through the increase of the cortical layers. 

In the Stictaceae there are a large number of widely-expanded forms, 
and as the attachment depends mostly on a somewhat short tomentum, 
strength is obtained here also by the thick plectenchymatous cortex of both 
surfaces. When areas denuded of tomentum and cortex occur, as in Lobaria 
pulmonaria, the under surface is not sensibly weakened, since the cortical 
tissue remains connected in a stout and firm reticulation. 

b. BY DEVELOPMENT OF VEINS OR NERVES. Certain ground lichens 
belonging to the Peltigeraceae have a wide spreading thallus often with 
very large lobes. The upper cortex is a many-layered plectenchyma, but 
the under surface is covered only by a loose felt of hyphae which branch 
out into a more or less dense tomentum. As the firm upper cortex continues 
to increase by intercalary growth from the branching upwards of hyphae 
from the meristematic gonidial zone, there occurs an extension of the upper 



STRATOSE THALLUS 97 

thallus with which the lower cannot keep pace 1 . A little way back from 
the edge, the result of the stretching is seen in the splitting asunder of the 
felted hyphae of the under surface, and in the consequent formation of a 
reticulate series of ridges known as the veins or nerves ; they represent the 
original tomentose covering, and are white, black or brown, according to the 
colour of the tomentum itself. The naked ellipsoid interstices show the 
white medulla, and, if the veins are wide, the colourless areas are correspond- 
ingly small. Rhizinae are formed on the nerves in several of the species, 
and anchor the thallus to the support. In Peltigera canina, the under surface 
is almost wholly colourless, the veins are very prominent (Fig. 55), and are 
further strengthened by the growth and branching of the parallel hyphae of 
which they are composed. They serve to strengthen the large and flabby 
thallus and form a rigid base for the long rhizinae by which the lichen clings 
to the grass or moss over which it grows. 

The most perfect development of strengthening nerves is to be found in 
HydrotJiyria venosa*, a rather rare water lichen that occurs in the streams of 
North America. It consists of fan-like lobes of thin structure, the cortex 
being only about one cell thick. The fronds are about 3 cm. wide and they 
are contracted below into a stalk which serves to attach the plant to the 
substratum. Several fronds may grow together in a dense tuft, the expanded 
upper portion floating freely in the water. Frequently the plants form a 
dense growth over the rocky beds of the stream. 

At the point where the stalk expands into the free erect frond, there 
arise a series of stout veins which spread upwards and outwards. They are 
definitely formed structures and not adaptations of pre-existing tissues : 
certain hyphae arise from the medulla at the contracted base of the frond, 
take a radial direction and, by increase, become developed into firm strands. 
The individual hyphae also increase in size, and the swelling of the nerve 
gives rise to a ridge prominent on both surfaces. They seldom anastomose 
at first but towards the tips they become smaller and spread out in delicate 
ramifications which unite at various points. There is no doubt, as Bitter 1 
points out, that the nerves function as strengthening tissues and preserve the 
frond from the strain of the water currents which would, otherwise, tear apart 
the delicate texture. 

1 Bitter 1899. 2 Sturgis 1890. 



7- 



9 8 



MORPHOLOGY 



III. RADIATE THALLUS 
i. CHARACTERS OF RADIATE THALLUS 

In the stratose dorsiventral thallus, there is a widely extended growing 
area situated round the free margins of the thallus. In the radiate thallus 
of the fruticose or filamentous lichens, growth is confined to an apical region. 
Attachment to the substratum is at one point only the base of the plant 
thus securing the exposure of all sides equally to light. The cortex 
surrounds the fronds, and the gonidia (mostly Protococcaceae) lie in a zone 
or in groups between the cortex and the medulla. It is the highest type of 
vegetative development in the lichen kingdom, since it secures the widest 
room for the gonidial layer, and the largest opportunity for photosynthesis. 

Shrubby upright lichens consist mostly of strap-shaped fronds, either 
simple or branched, which may be broadened to thin bands (Fig. 57) or 
may be narrowed and thickened till they are almost cylindrical. The fronds 
vary in length according to the species from a few millimetres upwards: 




Fig. 57. Roccellafuciformis DC. 



RADIATE THALLUS 



99 



those of Roccella have been found measuring 30 cm. in length ; those of 
Ramalina reticulata, the largest of all the American lichens, extend to con- 
siderably more. 

Lichens of filamentous growth are more or less cylindrical (Fig. 58). 
They are in some species upright and of moderate length^ but in a few 




Fig. 58. Usnea barbata Web. (S. H., Photo.}, 

pendulous forms they grow to a great length : specimens of Usnea longissima 
have been recorded that measured 6 to 8 metres from base to tip. 

The radiate type of thallus occurs in most of the lichen groups but most 
frequently in the Gymnocarpeae. In gelatinous Discolichens it is repre- 
sented in the Lichinaceae. It is rare among Pyrenocarpeae : there is one 
very minute British lichen in that series, Pyrenidium actinellum, and one 
from N. America, Pyrenothamnia, that are of fruticose habit. 

2. INTERMEDIATE TYPES OF THALLUS 

Between the foliose and the fruticose types, there are intermediate forms 
that might be, and often are, classified now in one group and now in the 
other. These are chiefly : Physcia (Anaptychia) dliaris, Ph. leucomelas and 
the species of Evernia. 

72 



100 



MORPHOLOGY 



In the two former the habit is more or less fruticose as the plants are 
affixed to the substratum at a basal point, but the fronds are decumbent and 
the internal structure is of the dorsi ventral type : there is an upper "fibrous" 
cortex of closely compacted parallel hyphae, a gonidial zone the gonidia 
lying partly in the cortex and partly among the loose hyphae of the 
medulla and a lower cortex formed of a weft of hyphae which also run 
somewhat parallel to the surface. Both species are distinguished by the 
numerous marginal cilia, either pale or dark in colour. These two lichens 
are greyish-coloured on the upper surface and greyish or whitish below. 

Evernia furfuracea with a basal attachment 1 , and with a partly horizontal 
and partly upright growth, has a dorsiventral thallus, dark greyish-green 
above and black beneath, with occasional rhizinae towards the base. The 
cortex of both surfaces belongs to the "decomposed" type; the gonidial 
zone lies below the upper surface, and the medullary tissue is of loose hyphae. 
In certain forms of the species isidia are abundant on the upper surface, 
a character of foliose rather than of fruticose lichens. E. furfuracea grows 
on trees and very frequently on palings. 




Fig. 5Q. Evernia prunastri Ach. (M. P., Photo.}. 
1 See p. 108. 



RADIATE THALLUS 101 

E. prunastri, the second species of the genus, is more distinctly upright in 
habit, with a penetrating basal hold-fast and upright strap-shaped branching 
fronds, light-greyish green on the "upper" surface and white on the other 
(Fig. 59). The internal structure is sub-radiate; both cortices are "decom- 
posed"; the gonidial zone consists of somewhat loose groups of algae, very 
constant below the "upper" surface, with an occasional group in the pith 
near to the lower cortex in positions that are more exposed to light. There 
is also a tendency for the gonidial zone to pass round the margin and spread 
some way along the under side. The medulla is of loose arachnoid texture 
and the whole plant is very limp when moist. It grows on trees, often in 
dense clusters. 

3. FRUTICOSE AND FILAMENTOUS THALLUS 

A. GENERAL STRUCTURE OF THALLUS 

The conditions of strain and tension in the upright plant are entirely 
different from those in the decumbent thallus, and to meet the new require- 
ments, new adaptations of structure are provided either in the cortex or in 
the medulla. 

CORTICAL STRUCTURES. With the exception of the distinctly plec- 
tenchymatous cortex, all the other types already described recur in fruticose 
lichens; in various ways they have been modified to provide not only covering 
but support to the fronds. 

a. The fastigiate cortex. This reaches its highest development in 
Roccella in which the branched hyphal tips, slightly clavate and thick-walled, 
lie closely packed in palisade formation at right angles to the main axis 
(Fig. 45). They afford not only bending power, but give great consistency 
to the fronds. The cortex is further strengthened in R. fuciformis* by the 
compact arrangement of the medullary hyphae that run parallel with the 
surface, and among which occur single thick-walled filaments. The plant 
grows on maritime rocks in very exposed situations ; and the narrow strap- 
shaped fronds, as stated above, may attain a length of 30 cm., though usually 
they are from 10 to i8cm. in height. The same type of cortex, but less 
highly differentiated, affords a certain amount of stiffness to the cylindrical 
much weaker fronds of Thamnolia. 

b. The fibrous cortex. This type is found in a number of lichens with 
long filamentous hanging fronds. It consists of parallel hyphae, rarely septate 
and rarely branched, but frequently anastomosing and with strongly thick- 
ened "sclerotic" walls. Such a cortex is the only strengthening element in 
Alectoria, and it affords great toughness and flexibility tc .the thong-like 

1 Darbishire 1808. 



102 



MORPHOLOGY 



thallus. It is also present in Ramalina (Alectoria) thrausta, a species with 
slender fronds (Fig. 60). 





Fig. 60. Alectoria thrausta Ach. A, transverse section of frond; 
a, cortex; b, gonidia; c, arachnoid medulla x 37. B, fibrous 
hyphae from longitudinal section of cortex, x 430 (after Brandt). 







II 

'!!' 






In Usnea longissima the cortex both of the fibrillose branchlets and of 
the main axis is fibrous, and is composed of narrow thick-walled hyphae 

which grow in a long spiral round 
the central strand. The hyphae 
become more frequently septate 
further back from the apex (Fig. 6l). 
Such a type of cortex provides an 
exceedingly elastic and efficient pro- 
tection for the long slender thallus. 

The same type of cortex forms 
the strengthening element in the 
fruticose or partly fruticose members 
of the family Physciaceae. One of 
\\\es>e.,Teloschistesflavicans, is a bright 
yellow filamentous lichen with a 
somewhat straggling habit. The 
fronds are very slender and are either 
cylindrical or slightly flattened. The 



vi 

it 



Usnea longissima Ach. 
sections of outer cortex. 
the middle portion of 
Schulte). 



Longitudinal 
A, near the apex; B, 
fibril, xjs^ (after 



RADIATE THALLUS 



103 



hyphae of the outer cortex are compactly fibrous; added toughness is 
given by the presence of some longitudinal strands of hyphae in the central 
pith. 

Another still more familiar grey lichen, Physcia ciliaris, has long flat 
branching fronds which, though dorsiventral in structure, are partly upright 
in habit. Strength is secured as in Teloschistes by the fibrous upper cortex. 
Other species of Physciae are somewhat similar in habit and in structure. 

In Dendrographa leucophaea, a slender strap-shaped rock lichen, Darbi- 
shire 1 has described the outer cortex as composed of closely compacted 
parallel hyphae resembling the strengthening cortex of Alectoria and very 
different from the fastigiate cortex of the Roccellae with which it is usually 
classified. 



B. SPECIAL STRENGTHENING STRUCTURES 

a. SCLEROTIC STRANDS. This form of strengthening tissue is charac- 
teristic of Ramalina. With the exception of R. thrausta (more truly an 
Alectoria} all the species have a rather weak cortical layer of branching 
intricate thick-walled hyphae, regarded by Brandt 2 as plectenchymatous, 
but more correctly by Hue 3 as "decomposed" on account of the gelatinous 
walls and diminishing lumen of the irregularly arranged cells. 

In R. evernioides, a plant with very wide flat almost decumbent fronds 
of soft texture, in R. ceruchis and in R. homalea there is a somewhat compact 
medulla which gives a slight stiffness to the thallus. The other species of 
the genus are provided with strengthening mechanical tissue within the 
cortex formed of closely united sclerotic hyphae that run parallel to the 
surface (Fig. 62). In a transverse section of the thallus, this tissue appears 





A B 

Fig. 62. Ramalina minuscula Nyl. A, transverse section 
of frond x 37; B, longitudinal strengthening hyphae of 
inner cortex x 430 (after Brandt). 

1 Darbishire 1895. 2 Brandt 1906. 3 Hue 1906. 



104 



MORPHOLOGY 



sometimes as a continuous ring which may project irregularly into the pith 
(R. calicaris) ; more frequently it is in the form of strands or bundles which 
alternate with the groups of gonidia (R. siliquosa, R. Curnozvii, etc.). In 
R. fraxinea these strands may be scarcely discernible in young fronds, though 
sometimes already well developed near the tips. Occasionally isolated strands 
of fibres appear in the pith (R. Curnowii\ or the sclerotic projections may 
even stretch across the pith to the other side (R. strepsilis} (Fig. 75 B). 

In the Cladoniae support along with flexibility is secured to the upright 
podetium by the parallel closely packed hyphae that form round the 
hollow cylinder a band called the "chondroid" layer from its cartilage-like 
consistency. 

b. CHONDROID AXIS. The central medullary tissue in Ramalina is, with 
few exceptions, a loose arachnoid structure ; often the fronds are almost 
hollow. In one species of Usnea, U. Taylori, found in polar regions, there 
is a similar loose though very circumscribed medullary and gonidial tissue 
in the centre of the somewhat cylindrical thallus, and a wide band of sclerotic 
fibres towards the cortex. 





Fig. 63 A. 
branch. 
longissi 



A, (Jinea barbata Web. Longitudinal section of filament with young adventitious 
, chondroid axis; /;, gonidial tissue; c, cortex, x too (after Schwendener). B, U. 
<na Ach. Hyphae from central axis x 525 (after Schulte . 



In all other species of Usnea the medulla itself is transformed into a 
strong central strand of long-celled thick-walled hyphae closely knit together 
by frequent anastomoses (Fig. 63 A). This central strand of the Usneas is 
known as the "chondroid axis." A narrow band of loose air-containing 
hyphae and a gonidial zone lie round the central axis between it and the 
outer cortex (Fig. 63 A, b). At the extreme apex, the external cortical hyphae 
grow in a direction parallel with the long axis of the plant, but further back, 
they branch out at right angles and become swollen and mostly "decom- 
posed " as in the cortex of Ramalina. 



RADIATE THALLUS 



105 



In Letharia (L. vulpina, etc.) the structure is midway between Ramalina 
and Usnea : the central axis is either a solid strand of chondroid hyphae or 
several separate strands. 





Fig. 63 B. Usnea lo ngissima Ach. A, transverse section of fibril x 85. B, a, chondroid axis; 
b, gonidial tissue; c, cortex x 525 (after Schulte). 

In three other genera with upright fruticose thalli, Sphaerophorus, Ar- 
gopsis and Stereocaulon, rigidity is maintained by a medulla approaching the 
chondroid type. In Sphaerophorus the species may have either flattened or 
cylindrical branching stalks, but in all of them, the centre is occupied by 
longitudinal strands of hyphae. Argopsis, a monotypic genus from Ker- 
guelen, has a cylindrical branching thallus with a strong solid axis; it is 
closely allied to Stereocaulon, a genus of familiar moorland lichens. The 
central tissue of the stalks in Stereocaulon is also composed of elongate, 
thick-walled conglutinate hyphae, formed into a strand which is, however, 
not entirely solid. 

C. SURVEY OF MECHANICAL TISSUES 

Mechanical tissues scarcely appear among fungi, except perhaps as 
stoutish cartilaginous hyphae in the stalks of some Agarics (Collybiae, etc.), 
or as a ring of more compact consistency round the central hyphae of 
rhizomorphic strands. It is practically a new adaptation of hyphal structure 
confined to lichens of the fruticose group, where there is the same require- 
ment as in the higher plants for rigidity, flexure and tenacity. 

Rigidity is attained as in other plants by groups or strands of mechanical 
tissue situated close to the periphery, as they are so arranged in Rama- 
lina and Cladonia; or the same end is achieved by a strongly developed 



io6 MORPHOLOGY 

fastigiate cortex as in Roccella. Bending strains to which the same lichens 
are subjected, are equally well met by the peripheral disposition of the 
mechanical elements. 

Tenacity and elasticity are provided for in the pendulous forms either 
by a fibrous cortex as in Alectoria, or by the chondroid axis in Usnea. 
Haberlandt 1 has recorded some interesting results of tests made by him as 
to the stretching capacity of a freshly gathered pendulous species in which 
the central strand was from -5 to I mm. thick. He found he could draw it 
out 100 to no per cent, of its normal length before it gave way. In an 
upright species the frond broke when stretched 60 to 70 per cent. In both 
of the plants tested, the central strand retained its elasticity up to 20 per 
cent, of stretching. The outer cortical tissue was cracked and broken in 
the experiments. Schulte 2 calculated somewhat roughly the tenacity of 
Usnea longissima and found that a piece of the main axis 8 cm. long carried 
up to 300 grms. without breaking. 

D. RETICULATE FRONDS 

In the upright radiate thallus, more especially among the Ramalinae, 
though also among Cladoniae\\h.&z has appeared a reticulate thallus resulting 
from the elongate splitting of the tissues, and due to unequal growth tension 
and straining of the gelatinous cortex when swollen with moisture. In 
several species of Ramalina, the strap-shaped frond is hollow in the centre ; 
and strands of strengthening fibres give rise to a series of cortical ridges. 
The thinner tissue between is frequently torn apart and ellipsoid openings 
appear which do not however pierce beyond the central hollow. Such breaks 
are irregular and accidental though occurring constantly in Ramalina 
fraxinea, R. dilacerata, etc. 

A more complete type of reticulation is always present in a Californian 
lichen, Ramalina reticulata, in which the large flat frond is a delicate open 
network from tip to base (Fig. 64). It grows on the branches of deciduous 
trees and hangs in crowded tufts up to 30 cm. or more in length. Usually 
it is so torn, that the real size attainable can only be guessed at. It is 
attached at the base by a spreading discoid hold-fast, and, in mature plants, 
consists of a stoutish main axis from which side branches are irregularly 
given off. These latter are firm at the base like the parent stalk, but soon 
they broaden out into very wide fronds. Splitting begins at the tips of the 
branches while still young ; they are then spathulate in form with a slightly 
narrower recurved tip, below which the first perforations are visible, small at 
first, but gradually enlarging with the growth of the frond. 

Ramalina reticulata is an extremely gelatinous lichen and the formation 
1 Haberlandt 1896. 2 Schulte 1904. See p. 120. 



I0 8 MORPHOLOGY 

of the network was supposed by Lutz 1 to be entirely due to the swelling of 
the tissues, or the imbibition of water, causing tension and splitting. A more 
exact explanation of the phenomenon is given by Peirce 2 : he found that it 
was due to the thickened incurved tip, which, on the addition of moisture, 
swells in length, breadth and thickness, causing it to bend slightly upwards 
and then curve backwards over the thallus, thus straining the part imme- 
diately behind. These various movements result in the splitting of the frond 
while it is young and the cortices are thin and weak. 

Peirce made a series of experiments to test the capacity of the tissues 
to support tensile strains. In a dry state, a piece of the lichen held a weight 
up to I50grms.; when wet it broke with a weight of 3Ogrms. It was also 
observed that the thickness of the frond doubled on wetting. 

E. ROOTING BASE IN FRUTICOSE LICHENS 

Fruticose and filamentous lichens are distinguished by their mode of 
attachment to the substratum : instead of a system of rhizinae or of hairs 
spread over a large area, there is usually one definite rooting base by which 
the plant maintains its hold on the support. 

Intermediate between the foliose and fruticose types of thallus are 
several species which are decumbent in habit, but which are attached at one 
(or sometimes more) definite points, with but little penetration of the under- 
lying substance. One such lichen, Evernia furfuracea, has been classified 
now as foliose, and again as fruticose. The earliest stage of the thallus is 
in the form of a rosette-like sheath which bears rhizinae on the under 
surface, very numerous at the centre of the sheath, but entirely wanting 
towards the periphery. A secondary thallus of strap-shaped rather narrow 
fronds rises from the sheath and increases by irregular dichotomous branch- 
ing. These branches, which are considered by Zopf 3 as adventitious, may 
also come into contact with the substratum and produce a few rhizinae at 
that point; or if the frond is more closely applied, the irritation thus 
produced causes a still greater outgrowth of rhizinae and the formation of 
a new base from which other fronds originate. These renewed centres of 
growth are not of very frequent occurrence; they were first observed and 
described by Lindau 4 in another species, Evernia prunastri, and were aptly 
compared by him to the creeping stolons of flowering plants. 

Evernia furfuracea grows frequently on dead wood, palings, etc., as well 
as on trees. E.prunastri grows invariably on trees, and has a more constantly 
upright fruticose' habit; in this species also, a basal sheath is present, and 
the attachment is secured by means of rhizoidal hyphae which penetrate 
deeply into the periderm of the tree, taking advantage of the openings 

1 Lutz 1894. 2 Peirce 1898. Zop f 1903. * Lindau 1895. 



RADIATE THALLUS 



109 



afforded by the lenticels. The sheath hyphae are continuous with the medul- 
lary hyphae of the frond, and gonidia are frequently enclosed in the tissues ; 
the sheath spreads to some extent over the surface of the bark, and round 
the base of the fronds, thus rendering the attachment of the lichen to the 
tree doubly secure. 

Among Ramalinae, the development of the base was followed by Brandt 1 
in one species, R. Landroensis, an arboreal lichen from S. Tyrol. A rosette- 
like sheath was formed consisting solely of strands of thick-walled hyphae 
which spread over the bark. There were no gonidia included in the tissue. 

A different type of attachment was found by Lilian Porter 2 in corti- 
colous Ramalinae R. fraxinea, R. fastigiata, and R. pollinaria. The lichens 
were anchored to the tree by strands of closely compacted hyphae longi- 
tudinally arranged and continuous with the cortical hyphae. These enter 
the periderm of the tree by cracks or lenticels, and by wedge action cause 
extensive splitting. The strands may also spread horizontally and give rise 
to new plants. The living tissues of the tree were thus penetrated and 
injured, and there was evidence that hypertrophied tissue was formed and 
caused erosion of the wood. 

Several Ramalinae R. siliquosa, R. Curnowii, etc. grow on rocks, 
often in extremely exposed situations, in isolated tufts or in crowded swards 
(Fig. 65). The separate tufts are not unfrequently connected at the base by 




Fig. 6;. Ramalina siliquosa A. L. Sm., on rocks, reduced (M. P., Photo.). 
1 Brandt 1906. 2 Porter 1916. 



IIO MORPHOLOGY 

a crustaceous thallus. It is possible also to see on the rock, here and there, 
small areas of compact thalline granules that have scarcely begun to put out 
the upright fronds. These granules are corticate on the upper surface and 
contain gonidia; from the lower surface, slender branching hyphae in rhizoid- 
like strands penetrate down between the inequalities and separable particles 
of the rock, if the formation is granitic. They frequently have groups of 
gonidia associated with them, and they continue to ramify and spread, the 
pure white filaments often enough enclosing morsels of the rock. The 
upright fronds are continuous with the base and are thus securely anchored 
to the substratum. 

On a smooth rock surface such as quartzite a continuous sward o*f Rama- 
Una growth is impossible. The basal hyphae being unable to penetrate the 
even surface of the rock, the attachment is slight and the plants are easily 
dislodged. They do however succeed, sometimes, in taking hold, and small 
groups of fronds arise from a crustaceous base which varies in depth from 
5 to i mm. The tissues of this base are very irregularly arranged : towards 
the upper surface loose hyphae with scattered groups of algae are traversed 
by strands of gelatinized sclerotic hyphae similar to the strengthening tissues 
of the upright fronds, while down below there are to be found not only 
slender hyphae, but a layer of gonidia visible as a white and green film on 
the rock when the overlying particles are scaled off. 

Darbishire 1 found that attachment to the substratum by means of a 
basal sheath was characteristic of all the genera of Roccellaceae. He looks 
on this sheath, which is the first stage in the development of the plant, as 
a primary or proto-thallus, analogous to the primary squamules of the 
Cladoniae, and he carries the analogy still further by treating the upright 
fronds as podetia. The sheath of the Roccellaceae varies in size but it is 
always of very limited extent; it is mainly composed of medullary hyphae, 
and gonidia may or may not be present. The whole structure is permanent 
and important, and is generally protected by a well-developed upper cortex 
similar in structure to that of the upright' thallus, i.e. of a fastigiate type. 
There is no lower cortex. 

The two British species of Roccella R. fuciformis and R. phycopsis 
grow on maritime rocks, the latter also occasionally on trees. In R. fuci- 
formis, the attaching sheath is a flat structure which slopes up a little round 
the base of the upright frond. It is about 2 mm. thick, the cortex occupying 
about 40 /A of that space; a few scattered gonidia are present immediately 
below. The remaining tissue of the sheath is composed of firmly wefted 
slender filaments. Towards the lower surface, there is a more closely com- 
pacted dark brown layer from which pass out the hyphae that penetrate 
the rock. 

1 Darbishire 1898. 



RADIATE THALLUS in 

The sheath of R. phycopsis is a small structure about 3 to 4 mm. in width 
and 1*5 mm. thick. A few gonidia may be found below the dense cortical 
layer, but they tend to disappear as the upright fronds become larger and 
the shade, in consequence, more dense. Lower down the hyphae take an 
intensely yellow hue; mixed with them are also some brown filaments. 
A somewhat larger sheath 7 to 8 mm. wide forms the base of R. tinctoria. 
In structure it corresponds as do those of the other species with the ones 
already described. 

In purely filamentous species such as Usnea there is also primary sheath 
formation : the medullary hyphae spread out in radiating strands which 
force their way wherever possible into the underlying substance; on trees 
they enter into any chink or crevice of the outer bark like wedges ; or they 
ramify between the cork cells which are split up by the mere growth pressure. 
By the vertical increase of the base, the fronds may be hoisted up and 
an intercalary basal portion may arise lacking both gonidia and cortical 
layer. Very frequently several bases are united and the lichen appears to 
be of tufted habit. 

A basal sheath provides a similar firm attachment for Alectoria jubata 
and allied species: these are slender mostly dark brown lichens which hang 
in tangled filaments from the branches of trees, rocks, etc. 

These attaching sheaths differ in function as well as in structure from 
the horizontal thallus of the Cladoniaceae. They may be more truly com- 
pared with the primary thallus of the red algae Dumontia and Phyllophora 
which are similarly affixed to the substratum, while upright fronds of 
subsequent formation bear the fructifications. 

IV. STRATOSE-RADIATE THALLUS 
i. STRATOSE OR PRIMARY THALLUS 

A. GENERAL CHARACTERISTICS 

This series includes the lichens of one family only, the Cladoniaceae, the 
genera of which are characterized by the twofold thallus, 
one portion being primary, horizontal and stratose, the 
other secondary and radiate, the latter an upright simple 
or branching structure termed a "podetium" which nar- 
rows above, or widens to form a trumpet-shaped cup or 
"scyphus" (Fig. 66). The apothecia are terminal on the 
pocletium or on the margins of the scyphi ; in a few species 
they are developed on the primary thallus. Some degree 
of primary thallus-formation has been demonstrated in all Fig. 66. Cladonia 

, , r ., , pyxidata Hoffrn. 

the genera, if not in all the species of the family. The Basa i sq uamule and 

^eiius Cladina was established to include those species podetium. a, apo- 
thecia; s, spermo- 
of Cladonia in which, it was believed, only a secondary gonia (after Krabbe). 




ii2 MORPHOLOGY 

podetial thallus was present, but Wainio 1 found in Cladonia sylvatica a 
granular basal crust and, in Cladonia uncialts, minute round scales with crenate 
margins measuring from -5 to I mm. in width. In some species (subgenus 
Cladina) the primary thallus is quickly evanescent, in others it is granular 
or squamulose and persistent. Where the basal thallus is so much reduced 
as to be practically non-existent, apothecia are rarely developed and soredia 
are absent Renewal of growth in these lichens is secured by the dispersal 
of fragments of the podetial thallus; they are torn off and scattered by the 
wind or by animals, and, if suitable conditions are met, a new plant arises. 

Cladonia squamules vary in size from very small scales as in Cl. uncialis 
to the fairly large foliose fronds of Cl.foliacea which extend to 5 cm. in length 
and about i cm. or more in width. It is interesting to note that when the 
primary thallus is well developed, the podetia are relatively unimportant 
and frequently are not formed. As a rule the squamules are rounded or 
somewhat elongate in form with entire or variously cut and crenate margins. 
They may be very insignificant and sparsely scattered over the substratum, 
or massed in crowded swards of leaflets which are frequently almost upright. 
In colour they are bluish-grey, yellowish or brownish above, and white 
beneath (red in Cl. miniata], frequently becoming very dark-coloured towards 
the rooting base. These several characteristics are specific and are often of 
considerable value in diagnosis. In certain conditions of shade or moisture, 
squamules are formed on the podetium ; they repeat the characters of the 
basal squamules of the species. 

B. TISSUES OF THE PRIMARY. THALLUS 

The stratose layers of tissue in the squamules of Cladonia are arranged 
as in other horizontal thalli. 

a. CORTICAL TISSUE. In nearly all these squamules the cortex is of 
the "decomposed" type. In a few species there is a plectenchymatous 
formation in Cl. nana, a Brazilian ground species, and in two New Zealand 
species, CL enantia f. dilatata and Cl. Neo-Zelandica. The principal growing 
area is situated all round the margins though generally there is more activity 
at the apex. Frequently there is a gradual perishing of the squamule at the 
base which counterbalances the forward increase. 

The upper surface in some species is cracked into minute areolae; the 
cracks, seen in section, penetrate almost to the base of the decomposed 
gelatinous cortex. They are largely due to alternate swelling and contraction 
of the gelatinous surface, or to extension caused, though rarely, by intercalary 
growth from the hyphae below. The surface is subject to weathering and 
peeling as in other lichens; but the loss is constantly repaired by the upward 
growth of the meristematic hyphae from the gonidial zone ; they push up 

1 Wainio 1880. 



STRATOSE-RADIATE THALLUS 113 

between the older cortical filaments and so provide for the expansion as 
well as for the renewal of the cortical tissue. 

b. GONIDIAL TISSUE. The gonidia consisting of Protococcaceous algae 
form a layer immediately below the cortex. Isolated green cells are not 
unfrequently carried up by the growing hyphae into the cortical region, but 
they do not long survive in this compact non-aerated tissue. Their empty 
membranes can however be picked out by the blue stain they take with 
iodine and sulphuric acid. 

Krabbe 1 has described the phases of development in the growing region : 
he finds that differentiation into pith, gonidial zone and cortex takes place 
some little way back from the edge. At the extreme apex the hyphae lie 
fairly parallel to each other; further back, they branch upwards to form the 
cortex, and to separate the masses of multiplying gonidia, by pushing 
between them and so spreading them through the whole apical tissue. The 
gonidia immediately below the upper cortex, where they are well-lighted, 
continue to increase and gradually form into the gonidial zone; those that 
lie deeper among the medullary hyphae remain quiescent, and before long 
disappear altogether. 

Where the squamules assume the upright position (as in Cladonia cei~vi- 
corms), there is a tendency for the gonidia to pass round to the lower 
surface, and soredia are occasionally formed. 

c. MEDULLARY TISSUE. The hyphae of the medulla are described by 
Wainio as having long cells with narrow lumen, and as being encrusted 
with granulations that may coalesce into more or less detachable granules; 
in colour they are mostly white, but pale-yellow in Cl.foliacea and blood-red 
in Cl. miniata, a subtropical species. They are connected at the base of the 
squamules with a filamentous hypothallus which penetrates the substratum 
and attaches the plant. In a few species rhizinae are formed, while in others 
the hyphae of the podetium grow downwards, towards and into the sub- 
stratum as a short stout rhizoid. 

d. SOREDIA. Though frequent on the podetia, soredia are rare on the 
squamules, and, according to Wainio 2 , always originate at the growing 
region, from which they spread over the under surface rather sparsely in 
Cl. cariosa, Cl. squamosa, etc., but abundantly in Cl. digitata and a few others. 
In some instances, they develop further into small corticate areolae on the 
under surface (Cl. cocci/era, Cl. pyxidata and Cl. squamosd). 

1 Krabbe 1891. 2 Wainio 1897. 



II4 MORPHOLOGY 

2. RADIATE OR SECONDARY THALLUS 

A. ORIGIN OF THE PODETIUM 

The upright podetium, as described by Wainio 1 and by Krabbe 2 , is a 
secondary product of the basal granule or squamule. It is developed from 
the hyphae of the gonidial zone, generally where a crack has occurred in the 
cortex and rather close to the base or more rarely on or near the edge of 
the squamule (Cl. verticillata, etc.). At these areas, certain meristematic 
gonidial hyphae increase and unite to form a strand of filaments below the 
upper cortex but above the gonidial layer, the latter remaining for a time 
undisturbed as to the arrangement of the algal cells. 

This initial tissue the primordium of the podetium continues to grow 
not only in width but in length: the basal portion grows downwards and 
at length displaces the gonidial zone, while the upper part as a compact 
cylinder forces its way through the cortex above, the cortical tissue, however, 
taking no part in its formation ; as it advances, the edges of the gonidial 
and cortical zones bend upwards and form a sheath distinguishable for some 
time round the base of the emerging podetium. 

Even when the primary horizontal thallus is merely crustaceous, the 
podetia take origin similarly from a subcortical weft of hyphae in an areola 
or granule. 

B. STRUCTURE OF THE PODETIUM 

a. GENERAL STRUCTURE. In the early stages of development the 
podetium is solid throughout, two layers of tissue being discernible the 
hyphae forming the centre of the cylinder being thick-walled and closely 
compacted, and the hyphae on the exterior loosely branching with numerous 
air-spaces between the filaments. 

In all species, with the exception of Cl. solida, which remains solid during 
the life of the plant, a central cavity arises while the podetium is still quite 
short (about i to i - 5 mm. in Cl. pyxidata and Cl. degenerans). The first 
indication of the opening is a narrow split in the internal cylinder, due to 
the difference in growth tension between the more free and rapid increase 
of the external medullary layer and the slower elongation of the chondroid 
tissue at the centre. The cavity gradually widens and becomes more com- 
pletely tubular with the upward growth of the podetium ; it is lined by 
the chondroid sclerotic band which supports the whole structure (Fig. 67). 

b. GONIDIAL TISSUE. In most species of Cladoniaceae, a layer of goni- 
dial tissue forms a more or less continuous outer covering of the podetium, 

1 Wainio 1880. 2 Krabbe 1891. 



STRATOSE-RADIATE THALLUS 







thus distinguishing it from the purely hyphal stalks of the apothecia in 
Caliciaceae. Even in the genus Baeomyces, 
while the podetia of some of the species 
are without gonidia, neighbouring species 
are provided with green cells on the up- 
right stalks clearly showing their true 
affinity with the Cladoniae. In one British 
species of Cladonia {Cl. caespiticia) the 
short podetium consists only of the fibrous 
chondroid cylinder, and thus resembles the 
apothecial stalk of Baeomyces rufus, but 
in that species also there are occasional 
surface gonidia that may give rise to 
squamules. 

Krabbe 1 concluded from his observa- 
tions that the podetial gonidia of Cladonia 
arrived from the open, conveyed by wind, 
water or insects from the loose sored ia that 
are generally so plentiful in any Cladonia 
colony. They alighted, he held, on the 
growing stalks and, being secured by the 
free-growing ends of the exterior hyphae, 
they increased and became an integral part of the podetium. In more 
recent times Baur 2 has recalled and supported Krabbe's view, but Wainio 3 , 
on the contrary, claims to have proved that in the earliest stages of the 
podetium the gonidia were already present, having been carried up from 
the gonidial zone of the primary thallus by the primordial hyphae. Increase 
of these green cells follows normally by cell-division or sporulation. 

Algal cells have been found to be common to different lichens, but in 
Cladoniae Chodat 4 claims to have proved by cultures that each species 
tested has a special gonidium, determined by him as a species of Cystococcus, 
which would render colonization by algae from the open much less probable. 
In addition, the fungal hyphae are specific, and any soredia (with their 
combined symbionts) that alighted on the podetium could only be utilized 
if they originated from the same species; or, if they were incorporated, the 
hyphae belonging to any other species would of necessity die off and be 
replaced by those of the podetium. 

c. CORTICAL TISSUE. In some species a cortex of the decomposed type 
of thick-walled conglutinate hyphae is present, either continuous over the 
whole surface of the podetium, as in Cl. gracilis (Fig. 68), or in interrupted 



stage of central tube and of podetial 
squamulesx 100 (after Krabbe). 



1 Krabbe t! 



2 Baur 1904. 



3 Wainio 1880. 



Chodat 1913. 

82 



MORPHOLOGY 




Fig. 68. Cladonia gracilis Hoffm. (S. H., Photo.). 




Fig. 69. Cladonia pyxtdata Hoftm. (S. H., Photo.] 



STRATOSE-RADIATE THALLUS 



117 



areas or granules as in Cl. pyxidala (Fig. 69) and others. In Cl. degenerans, 
the spaces between the corticated areolae are filled in by loose filaments 
without any green cells. CL rangiferina, Cl. sylvatica, etc. are non-corticate, 
being covered all over with a loose felt of intricate hyphae. 

In the section Clathrinae (Cl. retepora, etc.) the cortex is formed of 
longitudinal hyphae with thick gelatinous walls. 

d. SOREDlA. Frequently the podetium is coated in whole or in part by 
granules of a sorediate character coarsely granular in Cl. pyxidata, finely 
pulverulent in CL fimbriata. Though fairly constant to type in the different 
species, they are subject to climatic influences, and, when there is abundant 
moisture, both soredia and areolae develop into squamules on the podetium. 
A considerable number of species have thus a more or less densely squamu- 
lose "form" or "variety." 



C. DEVELOPMENT OF THE SCYPHUS 

Two types of podetia occur in Cladonia : those that end abruptly and 
are crowned when fertile by the apothecia or spermogonia (pycnidia), or if 
sterile grow indefinitely tapering gradually to a point (Fig. 70); and those 




Fig. 70. Cladonia furcata Schrad. Sterile thallus (S.H., Photo.']. 

that widen out into the trumpet-shaped or cup-like expansion called the 
scyphus (Fig. 69). Species may be constantly scyphiferous or as constantly 
ascyphous; in a few species, and even in individual tufts, both types of 
podetium may be present. 



n8 MORPHOLOGY 

Wainio 1 , who has studied every stage of development in the Cladoniae, 
has described the scyphus as originating in several different ways: 

a. FROM ABORTIVE APOTHECIA. In certain species the apothecium 
appears at a very early stage in the development of the podetium of which 
it occupies the apical region. Owing to the subsequent formation of the 
tubular cavity in the centre of the stalk, the base of the apothecium may 
eventually lie directly over the hollow space and, therefore, out of touch 
with the growing assimilating tissues; or even before the appearance of the 
tube, the wide separation between the primordium of the apothecium and 
the gonidia, entailing deficient nutrition, may have produced a similar effect. 
In either case central degeneration of the apothecium sets in, and the 
hypothecial filaments, having begun to grow radially, continue to travel in 
the same direction both outwards and upwards so that gradually a cup- 
shaped structure is evolved the amphithecium of the fruit without the 
thecium. 

The whole or only a part of the apothecium may be abortive, and the 
scyphus may therefore be entirely sterile or the fruits may survive at the 
edges. The apothecia may even be entirely abortive after a fertile com- 
mencement, but in that case also the primordial hyphae retain the primitive 
impulse not only to radial direction, but also to the more copious branching, 
and a scyphus is formed as in the previous case. It must also be borne in 
mind that the tendency in many Cladonia species to scyphiform has become 
hereditary. 

Baur 2 , in his study of Cl. pyxidata, has taken the view that the origin of 
the scyphus was due to a stronger apical growth of the hyphae at the 
circumference than over the central tubular portion of the podetium, and 
that considerable intercalary growth added to the expanding sides of the cup. 

Scyphi originating from an abortive apothecium are characteristic of 
species in which the base is closed (Wainio's Section Clausae\ the tissue in 
that case being continuous over the inside of the cup as in Cl. pyxidata, 
CL cocci/era and many others. 

b. FROM POLYTOMOUS BRANCHING. Another method of scyphus forma- 
tion occurs in Cl. amaurocrea and a few other species in which the branching 
is polytomous (several members rising from about the same level). Con- 
crescence of the tissues at the base of these branches produces a scyphus ; 
it is normally closed by a diaphragm that has spread out from the different 
bases, but frequently there is a perforation due to stretching. These species 
belong to the Section Perviae. 

c. FROM ARRESTED GROWTH. In most cases however where the 
scyphus is open as in Cl.furcata, Cl. sguamosa, etc., development of the cup 

1 Wainio 1897. 2 Baur 1904. 



STRATOSE-RADIATE THALLUS 119 

follows on cessation of growth, or on perforation at the summit of the 
podetium. Round this quiescent portion there rises a circle of minute 
prominences which carry on the apical development. As they increase in 
size, the spaces between them are bridged over by lateral growth, and the 
scyphus thus formed is large or small according to the number of these 
outgrowths. Apothecia or spermogonia may be produced at their tips, or 
the vegetative development may continue. Scyphi formed in this manner 
are also open or "pervious." 

d. GONIDIA OF THE SCYPHUS. Gonidia are absent in the early stages 
of scyphus formation when it arises from degeneration of the apical 
tissues, either fertile or vegetative ; but gradually they migrate from the 
podetium, from the base of young outgrowths, or by furrows at the edge, and 
so spread over the surface of the cup. Soredia may possibly alight, as 
Krabbe insists that they do, and may aid in colonizing the naked area. 
Their presence, however, would only be accidental ; they are not essential, 
and scyphi are formed in many non-sorediate species such as Cl. vertidllata. 
The cortex of the scyphus becomes in the end continuous with that of the 
podetium and is always similar in type. 

e. SPECIES WITHOUT SCYPHI. In species where the whole summit of 
the podetium is occupied by an apothecium, as in Cl. bellidiflora, no scyphus 
is formed. There is also an absence of scyphi in podetia that taper to a 
point. In those podetia the hyphae are parallel to the long axis and remain 
in connection with the external gonidial layer so that they are unaffected 
by the central cavity. Instances of tapering growth are also to be found 
in species that are normally scyphiferous such as Cl. fimbriata subsp. jil?u/a, 
and Cl. cornuta, as well as in species like Cl. rangiferina that are constantly 
ascyphous. 

The scyphus is considered by Wainio 1 to represent an advanced stage 
of development in the species or in the individual, and any conditions that 
act unfavourably on growth, such as excessive dryness, would also hinder 
the formation of this peculiar lichen structure. 

D. BRANCHING OF THE PODETIUM 

Though branching is a constant feature in many species, regular dicho- 
tomy is rare; more often there is an irregular form of polytomy in which one 
of the members grows more vigorously than the others and branches again, 
so that a kind of sympodium arises, as in Cl. rangiferina, Cl. sylvatica, etc. 

Adventitious branches may also arise from the podetium, owing to some 
disturbance of the normal growth, some undue exposure to wind or to too 

i Wainio 1897. 



120 MORPHOLOGY 

great light, or owing to some external injury. They originate from the 
gonidial tissue in the same way as does the podetium from the primary 
thallus; the parallel hyphae of the main axis take no part in their develop- 
ment. 

In a number of species secondary podetia arise from the centre of the 
scyphus constantly in Cl. verticillata and Cl. cervicornis, etc., accidentally 
or rarely in Cl. foliacea, Cl. pyxidata, CL fimbriata, etc. Wainio 1 has stated 
that they arise when the scyphus is already at an advanced stage of growth 
and that they are to be regarded as adventitious branches. 

The proliferations from the borders of the scyphus are in a different 
category. They represent the continuity of apical growth, as the edges of 
the scyphus are but an enlarged apex. These marginal proliferations thus 
correspond to polytomous branching. In many instances their advance is 
soon stopped by the formation of an apothecium and they figure more as 
fruit stalks than as podetial branches. 

E. PERFORATIONS AND RETICULATION OF THE PODETIUM 

Perforations in the podetial wall at the axils of the branches are constant 
in certain species such as Cl. rangiferina, CL uncialis, etc. They are caused 
by the tension of the branches as they emerge from the main stalk. 
A tearing of the tissue may also arise in the base of the scyphus, due to its 
increase in size, which causes the splitting of the diaphragm at the bottom 
of the cup. 

Among the Cladoniae the reticulate condition recurs now and again. 
In our native Cladonia cariosa the splitting of the podetial wall is a constant 
character of the species, the carious condition being caused by unequal 
growth which tears apart the longitudinal fibres that surround the central 
hollow. 

A more advanced type of reticulation arises in the group of the Clathrinae 
in which there is no inner chondroid cylinder. In Cladonia aggregata, in 
which the perforations are somewhat irregular, two types of podetia have 
been described by Lindsay 2 from Falkland Island specimens: those bearing 
apothecia are short and broad, fastigiately branched upwards and with 
reticulate perforations, while podetia bearing spermogonia are slender, elon- 
gate and branched, with fewer reticulations. An imperfect network is also 
characteristic of CL Sullivani, a Brazilian species. But the most marvellous 
and regular form of reticulation occurs in Cl. retepora, an Australian lichen 
(Fig. 71): towards the tips of the podetia the ellipsoid meshes are small, 
but they gradually become larger towards the base. In this species the 
outer tissue, though of parallel hyphae, is closely interwoven and forms 

1 Wainio 1897. 2 Lindsay 1859, P- 171- 



STRATOSE-RADIATE THALLUS 121 

a continuous growth at the edges of the perforations, giving an unbroken 
smooth surface and checking any irregular tearing. The enlargement of 
the walls is solely due to intercalary growth. The origin of the reticulate 
structure in the Clathrinae is unknown, though it is doubtless associated 




Fig. 71. Cladonia retepora Fr. From Tasmania. 

with wide podetia and rendered possible by the absence of an internal 
chondroid layer. The reticulate structure is marvellously adapted for the 
absorption of water: Cl. retepora, more especially, imbibes and holds moisture 
like a sponge. 



F. ROOTING STRUCTURES OF CLADONIAE 

The squamules of the primary thallus are attached, as are most squa- 
mules, to the supporting substance by strands of hyphae which may be 
combined into simple or branching rhizinae and penetrate the soil or the 
wood on which the lichen grows. There is frequently but one of these 
rooting structures and it branches repeatedly until the ultimate branchlets 
end in delicate mycelium. Generally they are grey or brown and are not 



122 ' MORPHOLOGY 

easily traced, but when they are orange-coloured, as according to Wainio 1 
they frequently are in Cladonia miniata and Cl. digitata, they are more 
readily observed, especially if the habitat be a mossy one. 

In Cl. alpicola it has been found that the rooting structure is frequently 
as thick as the podetium itself. If the podetium originates from the basal 
portion of the squamule, the hyphae from the chondroid layer, surrounding 
the hollow centre, take a downward direction and become continuous with 
the rhizoid. Should the point of insertion be near the apex of the squamule, 
these hyphae form a nerve within the squamule or along the under surface, 
and finally also unite with the rhizoid at the base, a form of rooting charac- 
teristic of Cl. cartilaginea, Cl. digitata and several other species. 

Mycelium may spread from the rhizinae along the surface of the sub- 
stratum and give rise to new squamules and new tufts of podetia, a method 
of reproduction that is of considerable importance in species that are 
generally sterile and that form no soredia. 

Many species, especially those of the section Cladina, soon lose all 
connection with the substratum, there being a continual decay of the lower 
part of the podetia. As apical growth may continue for centuries, the 
perishing of the base is not to be wondered at. 

G. HAPTERA 

The presence of haptera in Cladoniae has already been alluded to. They 
occur usually in the form of cilia or rhizinae 2 , but differ from the latter in 
their more simple regular growth being composed of conglutinate parallel 
hyphae. They arise on the edges of the squamules or of the scyphus, but 
in Cl. foliacea and Cl. ceratophylla they are formed at the points of the 
podetial branches (more rarely in Cl. cervicornis and Cl. gracilis). By the aid 
of these rhizinose haptera the squamule or branch becomes attached to any 
substance within reach. They also aid in the production of new individuals 
by anchoring some fragment of the thallus to a support until it has grown 
to independent existence and has produced new rhizinae or holdfasts. They 
are a very prominent feature of Cl. verticillaris f. penicillata in which they 
form a thick fringe on the edges of the squamules, or frequently grow out 
as branched cilia from the proliferations on the margins of the scyphus. 

H. MORPHOLOGY OF THE PODETIUM 

In the above account, the podetia have been treated as part of the 
vegetative thallus, seeing that, partly or entirely, they are assimilative and 
absorptive organs. This view does not, however, take into account their 
origin and development, in consideration of which Wainio 3 and later Krabbe 4 

1 Wainio 1897. 2 Wainio l897) p< 9 3 Wainio I8g0i 4 Krabbe ,g 9r> 



STRATOSE-RADIATE THALLUS 123 

considered them as part of the sporiferous organ. This view was also held 
by some of the earliest lichenologists: Necker 1 , for instance, constantly 
referred to the upright structure as "stipes"; Persoon 2 included it, under 
the term "pedunculus," as part of the "inflorescence" of the lichen, and 
Acharius 3 established the name "podetium" to describe the stalk of the 
apothecium in Baeomyces. 

Later lichenologists, such as Wallroth 4 , looked on the podetia as advanced 
stages of the thallus, or as forming a supplementary thallus. Tulasne 5 
described them as branching upright processes from the horizontal form, 
and Koerber 6 considered them as the true thallus, the primary squamule 
being merely a protothallus. By them and by succeeding students of lichens 
the twofold character of the thallus was accepted until Wainio and Krabbe 
by their more exact researches discovered the endogenous origin of the 
podetium, which they considered was conclusive evidence of its apothecial 
character: they claimed that the primordium of the podetium was homolo- 
gous with the primordium of the apothecium. Reinke 7 and Wainio are in 
accord with Krabbe as to the probable morphological significance of the 
podetium, but they both insist on its modified thalline character. Wainio 
sums up that: "the podetium is an apothecial stalk, that is to say an 
elongation of the conceptacle most frequently transformed by metamorphosis 
to a vertical thallus, though visibly retaining its stalk character." Sattler 8 , 
one of the most recent students of Cladonia, regards the podetium as evolved 
with reference to spore-dissemination, and therefore of apothecial character. 
His views are described and discussed in the chapter on phylogeny. 

Reinke and others sought for a solution of the problem in Baeomyces, 
one of the more primitive genera of the Cladoniaceae. The thallus, except 
in a few mostly exotic species, scarcely advances beyond the crustaceous 
condition; the podetia are short and so varied in character that species 
have been assigned by systematists to several different genera. In one of 
them, Baeomyces roseus, the podetium or stalk originates according to 
Nienburg 9 deep down in the medulla of a fertile granule as a specialized 
weft of tissue; there is no carpogonium nor trichogyne formed ; the hyphae 
that grow upward and form the podetium are generative filaments and give 
rise to asci and paraphyses. In a second species, B. rufus (Sphyridium\ the 
gonidial zone and outer cortex of a thalline granule swell out to form a 
thalline protuberance; the carpogonium arises close to the apex, and from 
it branch the generative filaments. Nienburg regards the stalk of B. roseus 
as apothecial and as representing an extension of the proper margin 10 (ex- 
cipulmn propriuwi), that of B. rufus as a typical vegetative podetium. 

1 Necker 1871. 2 Persoon 1794. 3 Acharius 1803. 4 Wallroth 1829, p. 61. 

8 Tulasne 1852. <* Koerber 1855. 7 Reinke 1894. 8 Sattler 1914. 

9 Nienburg 1908. 10 See p. 183. 



i2 4 MORPHOLOGY 

In the genus Cladonia, differentiation of the generative hyphae may 
take place at a very early stage. Wainio 1 observed, in CL caespiticia, a 
trichogyne in a still solid podetium only 90 /x in height; usually they appear 
later, and, where scyphi are formed, the carpogonium often arises at the 
edge of the scyphus. Baur 2 and Wolff 3 have furnished conclusive evidence 
of the late appearance of the carpogonium in CL pyxidata, Cl. degenerans, 
CL furcata and CL gracilis: in all of these species carpogonia with tricho- 
gynes were observed on the edge of well-developed scyphi. Baur draws the 
conclusion that the podetium is merely a vertical thallus, citing as additional 
evidence that it also bears the spermogonia (or pycnidia), though at the 
same time he allows that the apothecium may have played an important part 
in its phylogenetic development. He agrees also with the account of the 
first appearance of the podetium as described by Krabbe, who found that 
it began with the hyphae of the gonidial zone branching upwards in a quite 
normal manner, only that there were more of them, and that they finally 
pierced the cortex. Krabbe also asserted that in the early stages the podetia 
were without gonidia and that these arrived later from the open as colonists, 
in this contradicting Wainio's statement that gonidia were carried up from 
the primary thallus. 

It seems probable that the podetium as Wainio and Baur both have 
stated is homologous with the apothecial stalk, though in most cases it is 
completely transformed into a vertical thallus. If the view of their formation 
from the gonidial zone is accepted, then they differ widely in origin from 
normal branches in which the tissues of the main axis are repeated in the 
secondary structures, whereas in this vertical thallus, hyphae from the 
gonidial zone alone take part in the development. It must be admitted 
that Baur's view of the podetium as essentially thalline seems to be strength- 
ened by the formation of podetia at the centre of the scyphus, as "in CL 
verticillata, which are new structures and are not an elongation of the 
original conceptacular tissue. It can however equally be argued that the 
acquired thalline character is complete and, therefore, includes the possibility 
of giving rise to new podetia. 

The relegation of the carpogonium to a position far removed from the 
base or primordium of the apothecium need not necessarily interfere with 
the conception of the primordial tissue as homologous with the conceptacle; 
but more research is needed, as Baur dealt only with one species, CLpyxidata, 
and Gertrude Wolff confined her attention to the carpogonial stages at the 
edge of the scyphus. 

The Cladoniae require light, and inhabit by preference open moorlands, 
naked clay walls, borders of ditches, exposed sand-dunes, etc. Those with 
large and persistent squamules can live in arid situations, probably because 

1 Wainio 1897. 2 Baur 1904. 3 Wolff 1905. 



STRATOSE-RADIATE THALLUS 125 

the primary thallus is able to retain moisture for a long time 1 . When the 
primary thallus is small and feeble the podetia are generally much branched 
and live in close colonies which retain moisture. Sterile podetia are long- 
lived and grow indefinitely at the apex though the base as continually 
perishes and changes into humus. Wainio 2 cites an instance in which the 
bases of a tuft of Cl. alpestris had formed a gelatinous mass more than a 
decimetre in thickness. 



I. PlLOPHORUS AND STEREOCAULON 

These two genera are usually included in Cladoniaceae on account 
of their twofold thallus and their somewhat similar fruit formation. 
They differ from Cladonia in the development of the podetia which are 
not endogenous in origin as in that genus, but are formed by the growth 
upwards of a primary granule or squamule and correspond more nearly to 
Tulasne's conception of the podetium as a process from the horizontal 
thallus. In Pilophorus the primary granular thallus persists during the life 
of the plants; the short podetium is unbranched, and consists of a some- 
what compact medulla of parallel hyphae surrounded by a looser cortical 
tissue, such as that of the basal granule, in which are embedded the algal 
cells. The black colour of the apothecium is due to the thick dark hypo- 
thecium. 

Stereocaulon is also a direct growth from a short-lived primary squamule 3 . 
The podetia, called " pseudopodetia " by Wainio, are usually very much 
branched. They possess a central strand of hyphae not entirely solid, and 
an outer layer of loose felted hyphae in which the gonidia find place. A 
coating of mucilage on the outside gives a glabrous shiny surface, or, if 
that is absent, the surface is tomentose as in St. tomentosum. In all the 
species the podetia are more or less thickly beset with small variously 
divided squamules similar in form to the primary evanescent thallus. Gall- 
like cephalodia are associated with most of the species and aid in the work 
of assimilation. 

Stereocaulon cannot depend on the evanescent primary thallus for attach- 
ment to the soil. The podetia of the different species have developed various 
rooting bases: in St. ramulosum there is a basal sheath formed, in St. coral- 
hides a well-developed system of rhizoids 4 . 

1 Aigret 1901. 2 Wainio 1897. 3 Wainio 1890, p. 67. 4 Reinke 1895. 



126 MORPHOLOGY 

V. STRUCTURES PECULIAR TO LICHENS 

i. AERATION STRUCTURES 

A. CYPHELLAE AND PSEUDOCYPHELLAE 

The thallus of Stictaceae has been regarded by Nylander 1 and others as 
one of the most highly organized, not only on account of the size attained 
by the spreading lobes, but also because in that family are chiefly found 
those very definite cup-like structures which were named "cyphellae" by 
Acharius 2 . They are small hollow depressions about \ mm. or more in 
width scattered irregularly over the under surface of the thallus. 

a. HISTORICAL. Cyphellae were first pointed out by the Swiss botanist, 
Haller 3 . In his description of a lichen referable to Sticta fuliginosa he 
describes certain white circular depressions " to be found among the short 
brown hairs of the under surface." At a later date Schreber 4 made these 
" white excavated points " the leading character of his lichen genus Sticta. 

In urceolate or proper cyphellae, the base of the depression rests on the 
medulla; the margin is formed from the ruptured cortex and projects slightly 
inwards over the edge of the cup. Contrasted with these are the pseudo- 
cyphellae, somewhat roundish openings of a simpler structure which replace 
the others in many of the species. They have no definite margin ; the inter- 
nal hyphae have forced their way to the exterior and form a protruding 
tuft slightly above the surface. Meyer 5 reckoned them all among soredia; 
. but he distinguished between those in which the medullary hyphae became 
conglutinated to form a margin (true cyphellae) and those in which there 
was a granular outburst of filaments (pseudocyphellae). He also included 
a third type, represented in Lobaria pulmonaria on the under surface of 
which there are numerous non-corticate, angular patches where the pith is 
laid bare (Fig. 72). Delise 6 , writing about the same time on the Sticteae, 
gives due attention to their occurrence, classifying the various species of 
Sticta as cyphellate or non-cyphellate. 

Acharius had limited the name " cyphella " to the hollow urceolate bodies 
that had a well-defined margin. Nylander 7 at first included under that 
term both types of structure, but later 8 he classified the pulverulent " soredia- 
like " forms in another group, the pseudocyphellae. As a rule they bear no 
relation to soredia, and algae are rarely associated with the protruding 
filaments. Schwendener 9 , and later Wainio 10 , in describing Sticta aurata from 
Brazil, state, as exceptional, that the citrine-yellow pseudocyphellae of that 
species are sparingly sorediate. 

1 Nylander 1858, p. 63. ' 2 Acharius 1810, p. 12. 3 Haller 1768, p. 85. 

4 Schreber 1791, p. 768. 6 Meyer 1825, p. 148. s Delise 1822. 7 Nylander 1858, p. 14. 
8 Nylander 1860, p. 333. 9 Schwendener 1863, p. 169. 10 Wainio 1890, I. p. 183. 



STRUCTURES PECULIAR TO LICHENS 



127 



b. DEVELOPMENT OF CYPHELLAE. The cortex of both surfaces in the 
thallus of Sticta is a several-layered plectenchyma of thick-walled closely 




Fig. 72. Lobaria pulnionaria Hoffm. Showing pitted surface, a, under surface. 
Reduced (S. H., Photo.}. 

packed cells, the outer layer growing out into hairs on the under surface of 
most of the species. Where either cyphellae or pseudocyphellae occur, a 
more or less open channel is formed between the exterior and the internal 
tissues of the lichen. In the case of the cyphellae, the medullary hyphae 
which line the cup are divided into short roundish cells with comparatively 
thin -walls (Fig. 73). They form a tissue sharply differentiated from the 




Fig. 73. Sticta damaecornis Nyl. Transverse section 
of thallus with cyphella x 100. 

loose hyphae that occupy the medulla. The rounded cells tend to lie in 
vertical rows, though the arrangement in fully formed cyphellae is generally 



128 MORPHOLOGY 

somewhat irregular. The terminal empty cells are, loosely attached and as 
they are eventually abstricted and strewn over the inside of the cup they 
give to it the characteristic white powdery appearance. 

According to Schwendener 1 development begins by an exuberant growth 
of the medulla which raises and finally bursts the cortex; prominent cyphellae 
have been thus formed in Sticta damaecornis (Fig. 73). In other species 
the swelling is less noticeable or entirely absent. The opening of the cup 
measures usually about \ mm. across, but it may stretch to a greater width. 

c. PsEUDOCYPHELLAE. In these no margin is formed, the cortex is 
simply burst by the protruding filaments which are of the same colour 
yellow or white as the medullary hyphae. They vary in size, from a minute 
point up to 4 mm. in diameter. 

d. OCCURRENCE AND DISTRIBUTION. The genus Sticta is divided into 
two sections : (i) Eusticta in which the gonidia are bright-green algae, and 
(2) Stictina in which they are blue-green. Cyphellae and pseudocyphellae 
are fairly evenly distributed between the sections; they never occur together. 
Stizenberger 2 found that 36 species of the section Eusticta were cyphellate, 
while in 43 species pseudocyphellae were formed. In the section Stictina 
there were 38 of the former and only 31 of the latter type. Both sections of 
the genus are widely distributed in all countries, but they are most abundant 
south of the equator, reaching their highest development in Australia and 
New Zealand. 

In the British Isles Sticta is rather poorly represented as follows: 
\Eusticta (with bright-green gonidia). 

Cyphellate: 5. damaecornis. 

Pseudocyphellate: S. aurata. 

\Stictina (with blue-green gonidia). 

Cyphellate: S.fidiginosa, S. limbata, S. sylvatica, S. Dufourei. 

Pseudocyphellate: 5. intricata van Thouarsii, S. crocata. 

Structures resembling cyphellae, with an overarching rim, are sprinkled 
over the brown under surface of the Australian lichen, Heterodea Miilleri; 
the thallus is without a lower cortex, the medulla being protected by thickly 
woven hyphae. Heterodea was at one time included among Stictaceae, 
though now it is classified under Parmeliaceae. Pseudocyphellae are also 
present on the non-corticate under surface of Nephromium tomentosum, 
where they occur as little white pustules among the brown hairs; and the 
white impressed spots on the under surface of Cetraria Islandica and allied 
species, first determined as air pores by Zukal 3 , have also been described by 
Wainio 4 as pseudocyphellae. 

1 Schwendener 1863, p. 169. 2 Stizenberger 1895. 3 Zukal 1895, p. 1355. 4 Wainio 1909. 



STRUCTURES PECULIAR TO LICHENS 



129 



There seems no doubt that the chief function of these various structures 
is, as Schwendener 1 suggested, to allow a free passage of air to the assimi- 
lating gonidial zone. Jatta 2 considers them to be analogous to the lenticels 
of higher plants and of service in the interchange of gases expelling car- 
bonic acid and receiving oxygen from the outer atmosphere. It is remarkable 
that such serviceable organs should have been evolved in so few lichens. 




4. Parmelia exasperata Carroll. Ver- 
tical section of thallus. a, breathing- pores; 
l>, rhizoid. x 60 (after Rosendahl). 



B. BREATHING-PORES 

a. DEFINITE BREATHING-PORES. The cyphellae and pseudocyphellae 
described above are confined to the under surface of the thallus in those 
lichens where they occur. Distinct breathing-pores of a totally different 
structure are present on the upper 
surface of the tree-lichen, Parmelia 
aspidota (P. exasperata}, one of the 
brown-coloured species. They are 
somewhat thickly scattered as isidia- 
or cone-like warts over the lichen 
thallus (Fig. 74) and give it the char- 
acteristically rough or "exasperate" 
character. They are direct outgrowths 
from the thallus, and Zukal 3 , who dis- 
covered their peculiar nature and func- 
tion, describes them as being filled with a hyphal tissue, with abundant 
air-spaces, and in direct communication with the medulla ; gonidia, if 
present, are confined to the basal part. The cortex covering these minute 
cones, he further states, is very thin on the top, or often wanting, so that 
a true pore is formed which, however, is only opened after the cortex else- 
where has become thick and horny. Rosendahl 4 , who has re-examined these 
"breathing-pores," finds that in the early stage of their growth, near the 
margin or younger portion of the thallus, they are entirely covered by the 
cortex. Later, the hyphae at the top become looser and more frequently 
septate, and a fine net-work of anastomosing and intricate filaments takes 
the place of the closely cohering cortical cells. These hyphae are divided 
into shorter cells, but do not otherwise differ from those of the medulla. 
Rosendahl was unable to detect an open pore at any stage, though he 
entirely agrees with Zukal as to the breathing function of these structures. 
The gonidia of the immediately underlying zone are sparsely arranged and 
a few of them are found in the lower half of the cone; the hyphae of the 
medulla can be traced up to the apex. 



Schwendener 1863, p. 169. 



2 Jatta 1889, p. 4* 
4 Rosendahl 1907. 



3 Zukal 1895, p. 1357- 



S. L. 



130 



MORPHOLOGY 



Zukal 1 claims to have found breathing-pores in Cornicularia (Parmelid) 
tristis and in several other Parmeliae, notably 
in Parmelia stygia. The thallus of the latter 
species has minute holes or openings in the 
upper cortex, but they are without any definite 
form and may be only fortuitous. 

Zukal 1 published drawings of channels of 
looser tissue between the exterior and the 
pith in Oropogon Loxensis and in Usnea bar- 
bata. He considered them to be of definite 
service in aeration. The fronds of Ramalina 
dilacerata by stretching develop a series of 
elongate holes. Reinke 2 found openings in 
Ramalina Eckloni which pierced to the centre 
of the thallus, and Darbishire 3 has figured 
a break in the frond of another species, R. 
fraxinea (Fig. 75 A), which he has designated 
as a breathing-pore. Finally Brandt 4 , in his 
careful study of the anatomy of Ramalinae, 
has described as breathing- pores certain open 
areas usually of ellipsoid form in the compact 
cortex of several species: in R. strepsilis 
(Fig. 75 B) and R. Landroensis, and in the 
British species, R. siliquosa and R. fraxinea. These openings are however 
mostly rare and difficult to find or to distinguish from holes that may 
be due to any accident in the life of the lichen. It is noteworthy that 




Fig. 75 A. Ramalina fraxinea Ach. 
A, surface view of frond, a, air- 

res; />, young apothecia. x \i. 
. transverse section of part of 
frond, a, breathing- pore \f, strength- 
ening fibres, x 37 (after Brandt). 




Fig- 75 B- Ramalina strepsilis Zahlbr. Transverse section 
of part of frond showing distribution of: a, air-pores, and 
f, strengthening fibres, x 37 (after Brandt). 

Rosendahl found no further examples of breathing-pores in the brown 
Parmeliae that he examined in such detail. No other organs specially 
adapted for aeration of the thallus have been discovered. 

b. OTHER OPENINGS IN THE THALLUS. Lobaria is the only genus of 
Stictaceae in which neither cyphellae nor pseudocyphellae are formed ; but 
in two species, L. scrobiculata and L. pulmonaria, the lower surface is marked 

1 Zukal 1895. 2 Reinke 1895, p. 183. 3 Darbishire 1901. 4 Brandt 1906. 



STRUCTURES PECULIAR TO LICHENS 131 

with oblong or angular bare convex patches, much larger than cyphellae. 
They are exposed portions of the medulla, which at these spots has been 
denuded of the covering cortex. Corresponding with these bare spots there 
is a pitting of the upper surface. 

A somewhat similar but reversed structure characterizes Umbilicaria 
pustulata, which as the name implies is distinguished by the presence of 
pustules, ellipsoid swellings above, with a reticulation of cavities below. 
Bitter 1 in this instance has proved that they are due to disconnected centres 
of intercalary growth which are more vigorous on the upper surface and 
give rise to cracks in the less active tissue beneath. These cracks gradually 
become enlarged ; they are, as it were, accidental in origin but are doubtless 
of considerable service in aeration. 

In some Parmeliae there are constantly formed minute round holes, 
either right through the apothecia (P. cetrata, etc.), or through the thallus 
(P. pertusd). Minute holes are also present in the under cortex of Par- 
melia vittata and of P. enteromorpha, species of the subgenus Hypogymnia. 
Nylander 2 , who first drew attention to these holes of the lower cortex, 
described them as arising at the forking of two lobes ; but though they do 
occur in that position, they as frequently bear no relation to the branching. 
Bitter's 3 opinion is that they arise by the decay of the cortical tissues in 
very limited areas, from some unknown cause, and that the holes that pierce 
right through the thallus in other species may be similarly explained. 

Still other minute openings into the thallus occur in Parmelia vittata, 
P. obscurata and P. farinacea var. obscurascens. In the two latter the open- 
ings like pin-holes are terminal on the lobes and are situated exactly on 
the apex, between the pith and the gonidial zone; sometimes several holes 
can be detected on the end of one lobe. Further growth in length is checked 
by these holes. They appear more frequently on the darker, better illumi- 
nated plants. In Parmelia vittata the terminal holes are at the end of 
excessively minute adventitious branches which arise below the gonidial 
zone on the margin of the primary lobes. All these terminal holes are 
directed upwards and are visible from above. 

Bitter does not attribute any physiological significance to these very 
definite openings in the thallus. It has been generally assumed that they 
aid in the aeration of the thallus; it is also possible that they may be of 
service in absorption, and they might even be regarded as open water con- 
ductors. 

1 Bitter 1899. 2 Nylander i874 2 . 3 Bitter i9Oi 2 . 



9-2 



I32 MORPHOLOGY 

C. GENERAL AERATION OF THE THALLUS 

Definite structures adapted to secure the aeration of the thallus in a 
limited number of lichens have been described above. These are the breathing- 
pores of Parmelia exasperata and the cyphellae and pseudocyphellae of the 
Stictaceae, with which also may be perhaps included the circumscribed 
breaks in the under cortex in some members of that family. 

Though lichens are composed of two actively growing organisms, the 
symbiotic plant increases very slowly. The absorption of water and mineral 
salts must in many instances be of the scantiest and the formation of carbo- 
hydrates by the deep-seated chlorophyll cells of correspondingly small 
amount. Active aeration seems therefore uncalled for though by no means 
excluded, and there are many indirect channels by which air can penetrate 
to the deeper tissues. 

In crustaceous forms, whether corticate or not, the thallus is often deeply 
seamed and cracked into areolae, and thus is easily pervious to water and 
air. The growing edges and growing points are also everywhere more or 
less loose and open to the atmosphere. In the larger foliose and fruticose 
lichens, the soredia that burst an opening in the thallus, and the cracks 
that are so frequent a feature of the upper cortex, all permit of gaseous 
interchange. The apical growing point of fruticose lichens is thin and porous, 
and in many of them the ribs and veins of their channelled surfaces entail 
a straining of the cortical tissue that results in the formation of thinner 
permeable areas. Zukal 1 devoted special attention to the question of aeration, 
and he finds evidenceof air-passages through empty spermogonia and through 
the small round holes that are constant in the upper surface of certain foliose 
species. He claims also to have proved a system of air-canals right through 
the thallus of the gelatinous Collemaceae. Though his proof in this instance 
is somewhat unconvincing, he establishes the abundant presence of air in 
the massively developed hypothecium of Collema fruits. He found that the 
carpogonial complex of hyphae was always well supplied with air, and that 
caused him to view with favour the suggestion that the function of the 
trichogyne is to provide an air-passage. In foliose lichens, the under surface 
is frequently non-corticate, in whole or in part; or the cortex becomes 
seamed and scarred with increasing expansion, the growth in the lower 
layers failing to keep pace with that of the overlying tissues, as in Umbili- 
caria pustulata. 

It is unquestionable that the interior of the thallus of most lichens con- 
tains abundant empty spaces between the loose-lying hyphae, and that these 
spaces are filled with air. 

1 Zukal 1895, p. 1348. 



STRUCTURES PECULIAR TO LICHENS 133 

2. CEPHALODIA 

A. HISTORICAL AND DESCRIPTIVE 

The term " cephalodium" was first used by Acharius 1 to designate cer- 
tain globose apothecia (pycnidia). At a later date he applied it to the 
peculiar outgrowths that grow on the thallus of Peltigera aphthosa, already 
described by earlier writers, along with other similar structures, as " cor- 
puscula," " maculae," etc. The term is now restricted to those purely vege- 
tative gall-like growths which are in organic connection with the thallus of 
the lichen, but which contain one or more algae of a different type from the 
one present in the gonidial zone. They are mostly rather small structures, 
and they take various forms according to the lichen species on which they 
occur. They are only found on thalli in which the gonidia are bright-green 
algae (Chlorophyceae) and, with a few exceptions, they contain only blue- 
green (Myxophyceae). Cephalodia with bright-green algae were found by 
Hue 2 on two Parmeliae from Chili, in addition to the usual blue-green forms; 
the one contained Urococcus, the other Gloeocystis. Several with both types 
of algae were detected also by Hue 2 within the thallus of Aspicilia spp. 

Florke 3 in his account of German lichens described the cephalodia that 
grow on the podetia of Stereocaulon as fungoid bodies, "corpuscula fungosa." 
Wallroth 4 , who had made a special study of lichen gonidia, finally established 
that the distinguishing feature of the cephalodia was their gonidia which 
differed in colour from those of the normal gonidial zone. He considered 
that the outgrowths were a result of changes that had arisen in the epidermal 
tissues of the lichens, and, to avoid using a name of mixed import such as 
" cephalodia," he proposed a new designation, calling them " phymata " or 
warts. 

Further descriptions of cephalodia were given by Th. M. Fries 5 in his 
Monograph of Stereocaulon and Pilophorus\ but the greatest advance in 
the exact knowledge of these bodies is due to Forssell 6 who made a com- 
prehensive examination of the various types, examples of which occurred, 
he found, in connection with about TOO different lichens. Though fairly 
constant for the different species, they are not universally so, and are some- 
times very rare even when present, and then difficult to find. A striking 
instance of variability in their occurrence is recorded for Ricasolia amplis- 
sima (Lobaria laciniatd) (Fig. 76). The cephalodia of that species are 
prominent upright branching structures which grow in crowded tufts irregu- 
larly scattered over the surface. They are an unfailing and conspicuous 
specific character of the lichens in Europe, but are entirely wanting in North 
American specimens. 

1 Acharius 1803. 2 Hue 1904 and 1910. 3 Florke 1815, IV. p. 15. 

4 Wallroth 1825, p. 678. 5 Th. M. Fries 1858. 6 Forssell 1884. 



134 



MORPHOLOGY 



As cephalodia contain rather dark-coloured, blue-green algae, they are 
nearly always noticeably darker than the thalli on which they grow, varying 
from yellowish-red or brown in those of Lecanora gelida to pale-coloured in 




Fig. 76. Ricasolia amplissima de Not. (Lobaria ladniata Wain.) on oak, reduced. The dark 
patches are tufts of branching cephalodia (A. Wilson, Photo.}. 

Lecidea consentiens^ , a darker red in Lecidea panaeola and various shades 
of green, grey or brown in Stereocanlon, Lobaria (Ricasolia}, etc. They form 
either flat expansions of varying size on the upper surface of the thallus, 
rounded or wrinkled wart-like growths, or upright branching structures. 
On the lower surface, where they are not unfrequent, they take the form of 
small brown nodules or swellings. In a number of species packets of blue- 
green algae surrounded by hyphae are found embedded in the thallus, 
either in the pith or immediately under the cortex. They are of the same 
nature as the superficial excrescences and are also regarded as cephalodia. 

1 Leigh ton 1869. 



STRUCTURES PECULIAR TO LICHENS 135 

B. CLASSIFICATION 
Forssell has drawn up a classification of these structures, as follows : 

I. CEPHALODIA VERA. 

1. Cephalodia epigena (including perigena) developed on the upper 
outer surface of the thallus, which are tuberculose, lobulate, clavate or 
branched in form. These are generally corticate structures. 

2. Cephalodia hypogena which are developed on the under surface 
of the thallus; they are termed "thalloid" if they are entirely superficial, 
and "immersed" when they are enclosed within the tissues. They are non- 
corticate though surrounded by a weft of hyphae. Forssell further includes 
here certain placodioid (lobate), granuliform and fruticose forms which 
develop on the hypothallus of the lichen, and gradually push their way up 
either through the host thallus, or, as in Lecidea panaeola, between the thalline 
granules. 

Nylander 1 arranged the cephalodia known to him in three groups: 
(i) Ceph. epigena, (2) Ceph. hypogena and (3) Ceph. endogena. Schneider 2 
still more simply and practically describes them as Ectotrophic (external), 
and Endotrophic (internal). 

II. PSEUDOCEPHALODIA. 

These are a small and doubtful group of cephalodia which are apparently 
in very slight connection with the host thallus, and show a tendency to 
independent growth. They occur as small scales on Solorina bispora 3 and 
vS". spongiosa and also on Lecidea pallida. Forssell has suggested that the 
cephalodia of Psoroma hypnorum and of Lecidea panaeola might also be in- 
cluded under this head. 

Forssell and others have found and described cephalodia in the following 
families and genera: 

Sphaerophoraceae. 

Sphaerophorus (S. stereocauloides). 

Lecideaceae. 

Lecidea (L. panaeola, L, consentiens, L. pelobotrya, etc.). 

Cladoniaceae. 

Stereocaulon, Pilophorus and Argopsis. 

Pannariaceae. 

Psoroma (P. hypnoruiri). 
Peltigeraceae. 

Peltigera (Peltidea), Nephroma and Solorina. 

1 Nylander 1878. 2 Schneider 1897. 3 Hue 1910. 



i 3 6 MORPHOLOGY 

Stictaceae. 

Lob aria, Sticta. 

Lecanoraceae. 

Lecania (L. lecanorina), Aspicilia 1 . 

Physciaceae. 

Placodium bicolor*. 

C. ALGAE THAT FORM CEPHALODIA 

The algae of the cephalodia belong mostly to genera that form the 
normal gonidia of other lichens. They are: 

Stigonema, in Lecanora gelida, Stereocaulon, Pilophorus robustus, and 
Lecidea pelobotrya. 

Scytonema, a rare constituent of cephalodia. 

Nostoc.. the most frequent gonidium of cephalodia. It occurs in those 
of the genera Sticta, Lobaria, Peltigera, Nephroma, Solorina and Psoroma; 
occasionally in Stereocaulon and in Lecidea pallida. 

Lyngbya and Rivularia, rarely present, the latter in Sticta oregana*. 

Chroococcus and Gloeocapsa, also very rare. 

Scytonema, Chroococcus, Gloeocapsa and Lyngbya are generally found 
in ^combination with some other cephalodia-building alga, though Nylander 4 
found Scytonema alone in the lobulate cephalodia of Sphaerophorus stereo- 
cauloides, a New Zealand lichen, and the only species of that genus in which 
cephalodia are developed; and Hue 1 records Gloeocapsa as forming internal 
cephalodia in two species of Aspicilia, Bornet 5 found Lyngbya associated 
with Scytonema in the cephalodia of Stereocaulon ramulosum, and, in the 
same lichen, Forssell 6 found, in the several cephalodia of one specimen, 
Nostoc, Scytonema, and Lyngbya, while, in those of another, Scytonema and 
Stigonema were present. In the latter instance these algae were living free 
on the podetium. Forssell 6 also determined two different algae, Gloeocapsa 
magma and Chroococcus tttrg-idus,preseni in a cephalodium on Lecidea panaeola 
var. elegans. 

As a general rule only one kind of alga enters into the formation of the 
cephalodia of any species or genus. A form of Nostoc, for instance, is in- 
variably the gonidial constituent of these bodies in the genera, Lobaria, Sticta, 
etc. In other lichens different blue-green algae, as noted above, may occupy 
the cephalodia even on the same specimen. Forssell finds alternative algae 
occurring in the cephalodia of: 

Lecanora gelida and Lecidea illita contain either Stigonema or Nostoc; 

Lecidea panaeola, with Gloeocapsa, Stigonema or Chroococcus; 

1 Hue 1910. 2 Tuckerman 1875. 3 Schneider 1897, p. 58. * Nylander 1869. 

5 Bornet 1873, p. 72. Forssell 1885, p. 24. 



STRUCTURES PECULIAR TO LICHENS 137 

Lecidea pelobotrya, with Stigonema or Nostoc; 

Pilophorus robustus, with Gloeocapsa, Stigonema, or Nostoc. 




Fig. 77. Lecanora gelida Ach. #, lobate cephalodia 
x 1 2 (after Zopf ). 

Riddle 1 has employed cephalodia with their enclosed algae as diagnostic 
characters in the genus Stereocaulon. When the alga is Stigonema, as in 
5. pascJiale, etc., the cephalodia are generally very conspicuous, grey in 
colour, spherical, wrinkled or folded, though sometimes black and fibrillose 
(S. denudatuni). Those containing Nostoc are, on the contrary, minute and 
are coloured verdigris-green (S. tomentosum and 5. alpinuni). 

Instances are recorded of algal colonies adhering to, and even penetrating, 
the thallus of lichens, but as they never enter into relationship with the 
lichen hyphae, they are antagonistic rather than symbiotic and have no 
relation to cephalodia. 

D. DEVELOPMENT OF CEPHALODIA 

a. EcTOTROPHIC. Among the most familiar examples of external cepha- 
lodia are the small rather dark-coloured warts or swellings that are scattered 
irregularly over the surface of Peltigera (Peltidea) aphthosa. This lichen has 
a grey foliose thallus of rather large sparingly divided lobes; it spreads 
about a hand-breadth or more over the surface of the ground in moist up- 
land localities. The specific name " aphthosa " was given by Linnaeus to 

1 Riddle 1910. 



i 3 8 MORPHOLOGY 

the plant on account of the supposed resemblance of the dotted thallus to 
the infantile ailment of " thrush." Babikoff l has published an account of 
the formation and development of these Peltidea cephalodia. He determined 
the algae contained in them to be Nostoc by isolating and growing them on 
moist sterilized soil. He observed that the smaller, and presumably younger, 
excrescences were near the edges of the lobes. The cortical cells in that 
position grow out into fine septate hairs that are really the ends of growing 
hyphae. Among the hairs were scattered minute colonies of Nostoc cells 
lying loose or so closely adhering to the hairs as to be undetachable (Fig. 

78 A). In older stages the hairs, evi- 
dently stimulated by contact with the 
Nostoc, had increased in size and sent 
out branches, some of which penetrated 
the gelatinous algal colony; others, 
spreading over its surface, gradually 
formed a cortex continuous with that 
of the thallus. The alga also increased, 
and the structure assumed a rounded or 
lentiform shape. The thalline cortex 
immediately below broke down, and 
the underlying gonidial zone almost 

Fig. 78 A. Hairs of Peltigera aphthosaVIi\\&. 7 S S 

associated with Nostoc colony much mag- wholly died off and became absorbed. 
nified (after Babikoff). The hyphae of the cephalodium had 

meanwhile penetrated downwards as root-like filaments, those of the thallus 
growing upwards into the new overlying tissue (Fig. 78 B). The foreign 
alga has been described as parasitic, as it draws from the lichen hyphae the 
necessary inorganic food material; but it might equally well be considered 
as a captive pressed into the service of the lichen to aid in the work of assi- 
milation or as a willing associate giving and receiving mutual benefit. 

Th. M. Fries 2 had previously described the development of the cephalodia 
in Stereocaulon but failed to find the earliest stages. He concluded from his 
observations that parasitic algae were common in the cortical layer of the 
lichens, but only rarely formed the " monstrous growths " called cephalodia. 

b. ENDOTROPHIC. Winter 3 examined the later stages of internal cepha- 
lodine formation in a species of Sticta. The alga, probably a species of 
Rivularia, which gives origin to the cephalodia, may be situated immediately 
below the upper cortex, in the medullary layer close to the gonidial zone, 
or between the pith and the under cortex. The protuberance caused by the 
increasing tissue, which also contains the invading alga, arises accordingly 
either on the upper or the lower surface. In some cases it was found that 
the normal gonidial layer had been pushed up by the protruding cephalodium 
1 Babikoff 1878. 2 Th. M. Fries 1866. 3 Winter 1877. 




STRUCTURES PECULIAR TO LICHENS 



139 



and lay like a cap over the top. The cephalodia described by Winter are 
endogenous in origin, though the mature body finally emerges from the 
interior and becomes either epigenous or hypogenous. Schneider 1 has fol- 
lowed the development of a somewhat similar endotrophic or endogenous type 




Fig. 788. Peltigera aphthosa Willd. Vertical section of thai lus and 
cephalodium x 480 (after Babikoff). 

in Sticta oregana due also to the presence of a species of Rivularia. How 
the alga attained its position in the medulla of the thallus was not observed. 
Both the algal cells of internal cephalodia and the hyphae in contact 
with them increase vigorously, and the newly formed tissue curving upwards 
or downwards appears on the outside as a swelling or nodule varying in 
size from that of a pin-head to a pea. On the upper surface the gonidial 
zone partly encroaches on the nodule, but the foreign alga remains in the 
centre of the structure well separated from the thalline gonidia by a layer 
of hyphae. The group is internally divided into small nests of dark-green 
algae surrounded by strands of hyphae (Fig. 79). The swellings, when they 




Fig. 79. Nephroma expallidum Nyl. Vertical section 
of thallus. a, endotrophic cephalodium x 100 (after 
Forssell). 

1 Schneider 1807. 



1 40 MORPHOLOGY 

occur on the lower surface of the lichen, correspond to those of the upper 
in general structure, but there is no intermixture of thalline gonidia. That 
Nostoc cells can grow and retain the power to form chlorophyll in adverse 
conditions was proved by Etard and Bouilhac 1 who made a culture of the 
alga on artificial media in the dark, when there was formed a green pigment 
of chlorophyll nature. 

Endotrophic cephalodia occur in many groups of lichens' Hue 2 states 
that he found them in twelve species of Aspicilia. As packets of blue-green 
algae they are a constant feature in the thallus of Solorinae. The species of 
that genus grow on mossy soil in damp places, and must come frequently 
in contact with Nostoc colonies. In Solorina crocea an interrupted band of 
blue-green algae lies below the normal gonidial zone and sometimes replaces 
it a connecting structure between cephalodia and a true gonidial zone. 

c. PSEUDOCEPHALODIA. Under this section have been classified those 
cephalodia that are almost independent of the lichen thallus though to some 
extent organically connected with it, as for instance that of Lecidea panaeola 
which originate on the hypothallus of the lichen and maintain their position 
between the crustaceous granules. 

The cephalodia of Lecanora gelida, as described by Sernander 3 , might 
also be included here. He watched their development in their native habitat, 
an exposed rock-surface which was richly covered with the lichen in all 
stages of growth. Two kinds of thallus, the one containing blue-green algae 
(Chroococcus}, the other bright-green, were observed on the rock in close 
proximity. At the point of contact, growth ceased, but the thallus with 
bright-green algae, being the more vigorous, was able to spread round and 
underneath the other and so gradually to transform it to a superficial flat 
cephalodium. All such thalli encountered by the dominant lichen were 
successively surrounded in the same way. The cephalodium, growing more 
slowly, sent root-like hyphae into the tissue of the underlying lichen, and 
the two organisms thus became organically connected. Sernander considers 
that the two algae are antagonistic to each other, but that the hyphae can 
combine with either. 

The pseudocephalodia of Usnea species are abortive apothecia; they are 
surrounded at the base by the gonidial zone and cortex of the thallus, and 
they contain no foreign gonidia. 

E. AUTOSYMBIOTIC CEPHALODIA 

Bitter 4 has thus designated small scales, like miniature thalli, that develop 
constantly on the upper cortex of Peltigera lepidophora, a small lichen not 
uncommon in Finland, and first recorded by Wainio as a variety of Peltigera 
1 Etard and Bouilhac 1898. 2 Hue I9 , o 3 Sernander 1907. * Bitter 1904. 



STRUCTURES PECULIAR TO LICHENS 141 

canina. The alga contained in the scales is a blue-green Nostoc similar to 
the gonidia of the thallus. Bitter 1 described the development as similar to 
that of the cephalodia of Peltigera aphthosa, but the outgrowths, being lobate 
in form, are less firmly attached and thus easily become separated and dis- 
persed ; as the gonidia are identical with those of the parent thallus they 
act as vegetative organs of reproduction. 

Bitter's work has been criticized by Linkola 2 who claims to have dis- 
covered by means of very thin microtome sections that there is a genetic 
connection between the scales and the underlying thallus, not only with the 
hyphae, as in true cephalodia, but with the algae as well, so that these out- 
growths should be regarded as isidia. 

In the earliest stages, according to Linkola, a small group of algae may 
be observed in the cortical tissue of the Peltigera apart from the gonidial 
zone and near the upper surface. Gradually a protruding head is formed 
which is at first covered over with a brown cortical layer one cell thick. The 
head increases and becomes more lobate in form, being attached to the thallus 
at the base by a very narrow neck and more loosely at other parts of the 
scale. In older scales, the gonidia are entirely separated from those of the 
thallus, and a dark-brown cortex several cells in thickness covers over the 
top and sides; there is a colourless layer of plectenchyma beneath. At this 
advanced stage the scales are almost completely superficial and correspond 
with the cephaloidal rather than with the isidial type of formation. The 
algae even in the very early stages are distinct from the gonidial zone and 
the whole development, if isidial, must be considered as somewhat abnormal. 

3. SOREDIA 

A. STRUCTURE AND ORIGIN OF SOREDIA 

Soredia are minute separable parts of the lichen thallus, and are com- 
posed of one or more gonidia which are clasped and surrounded by the 
lichen hyphae (Fig. 80). They occur on the sur- 
face or margins of the thallus of a fairly large 
number of lichens either in a powdery excrescence 
or in a pustule-like body comprehensively termed 
a "soralium" (Fig. 81). The soralia vary in form Fig. 80. Soredia. a, of Phystia 
and dimensions according to the species. Each fuiveruitntaXyl.-b, tiRama- 

hnafartnacea Ach. x 600. 

individual soredium is capable of developing into 

a new plant; it is a form of vegetative reproduction characteristic of lichens. 

Acharius 3 gave the name " soredia " to the powdery bodies with reference 

to their propagating function; he also interpreted the soredium as an "apo- 

thecium of the second order." But long before his time they had been 

1 Bitter 1904. 2 Linkola 1913. 3 Acharius, 1798, p. xix, and 1810, pp. 8 and 10. 




I4 2 MORPHOLOGY 

observed and commented on by succeeding botanists: first by Malpighi 1 
who judged them to be seeds, he having seen them develop new plants; by 




Fig. 81. Vertical section of young soralium of Evernia Jurfuracea 
var. soralifera Bitter x 60 (after Bitter). 

Micheli 2 who however distinguished between the true fruit and those seeds; 
and by Linnaeus 3 who considered them to be the female organs of the 
plant, the apothecia being, as he then thought, the male organs. Hedwig 4 , 
on the other hand, regarded the apothecia as the seed receptacles and the 
soredia as male bodies. Sprengel's 5 statement that they were "a subtile 
germinating powder mixed with delicate hair-like threads which take the 
place of seeds" established finally their true function. Wallroth 6 , who was 
the first really to investigate their structure and their relation to the parent 
plant, recognized them as of the same type as the "brood-cells" or gonidia; 
and as the latter, he found, could become free from the thallus and form a 
green layer on trees, walls, etc., in shady situations, so the soredia also 
could become free, though for a time they remained attached to the lichen 
and were covered by a veil, i.e. by the surrounding hyphal filaments. Koer- 
ber 7 also gave much careful study to soredia, their nature and function. As 
propagating organs he found they were of more importance than spores, 
especially in the larger lichens. 

According to Schwendener 8 , the formation of soredia is due to increased 
and almost abnormal activity of division in the gonidial cell; the hyphal 
filament attached to it also becomes active and sends out branches from the 
cell immediately below the point of contact which force their way between 
the newly divided gonidia and finally surround them. A soredial "head" 

1 Malpighi, 1686, p. 50, pi. 27, fig. 106. 2 Micheli 1729, pp. 73, 74. 3 Linnaeus 1737, p. 325. 
4 Hedwig 1798. 6 Sprengel 1807, Letter xxili. 6 Wallroth 1825, I. p. 595. 

7 Koerber 1841. 8 Schwendener 1860. 



STRUCTURES PECULIAR TO LICHENS 



of smaller or larger size is thus gradually built up on the stalk filament or 
filaments, and is ultimately detached by the breaking down of the slender 
support. 

a. SCATTERED SOREDIA. The simplest example of soredial formation 
may be seen on the bark of trees or on palings when the green coating of 
algal cells is gradually assuming a greyish hue caused by the invasion of 
hyphal lichenoid growth. This condition is generally referred to as " leprose " 
and has even been classified as a distinct genus, Lepra or Lepraria. 
Somewhat similar soredial growth is also associated with many species of 
Cladonia, the turfy soil in the neighbourhood of the upright podetia being 
often powdered with white granules. Such soredia are especially abundant 
in that genus, so much so, that Meyer 1 , Krabbe 2 and others have maintained 
that the spores take little part in the propagation of species. The under 
side of the primary thallus, but more frequently the upright podetia, are 
often covered with a coating of soredia, either finely furfuraceous, or of larger 
growth and coarsely granular, the size of the soredia depending on the 
number of gonidia enclosed in each " head." 

Soredia are only occasionally present on the apothecial margins: the 
rather swollen rims in Lobaria scrobiculata are sometimes powdery-grey, and 
Bitter 3 has observed soredia, or rather soralia, on the apothecial margins of 
Parmelia vittata; they are very rare, however, and are probably to be ex- 
plained by excess of moisture in the surroundings. 

b. ISIDIAL SOREDIA. In a few lichens soredia arise by the breaking 
down of the cortex at the tips of the thalline outgrowths termed "isidia." 
In Parmelia verruculifera, for instance, where the coralloid isidia grow in 
closely packed groups or warts, the upper part of the isidium frequently 
becomes soredial. In that lichen the younger parts of the upper cortex 
bear hairs or trichomes, and the individual soredia are also adorned with 
hairs. The somewhat short warted 

isidia of P. subaurifera may become 
entirely sorediose, and in P.farinacea 
the whole thallus is covered with isidia 
transformed into soralia. The trans- 
formation is constant and is a distinct 
specific character. Bitter 3 considers 
that it proves that no sharp distinction 
exists between isidia and soralia, at 
least in their initial stages. 

c. SOREDIA AS BUDS. Schwen- 
dener 4 has described soredia in the 




Fig. 82. Usttea barbata Web. Longitudinal 
section of filament and base of "soredial" 
branch x 40 (after Schwendener). 



1 Meyer 1825, p. 170. 2 Krabbe 1891.. 



Bitter 1901. 4 Schwendener 1860, p. 137. 



i 44 MORPHOLOGY 

genus Usnea which give rise to new branches. Many of the species in that 
genus are plentifully sprinkled with the white powdery bodies. A short 
way back from the apex of the filament the separate soredia show a tendency 
to apical growth and might be regarded as groups of young plants still 
attached to the parent branch. One of these developing more quickly 
pushes the others aside and by continued growth fills up the soredial 
opening in the cortex with a plug of tissue; finally it forms a complete 
lateral branch. Schwendener calls them "soredial" branches (Fig. 82) to 
distinguish them from the others formed in the course of the normal 
development. 

B. SORALIA 

In lichens of foliose and fruticose structure, and in a few crustaceous 
forms, the soredia are massed together into the compact bodies called soralia, 
and thus are confined to certain areas of the plant surface. The simpler 
soralia arise from the gonidial zone below the cortex by the active division 
of some of the algal cells. The hyphae, interlaced with the green cells, are 
thin-walled and are, as stated by Wainio 1 , still in a meristematic condition ; 
they are thus able readily to branch and to form new filaments which clasp 
the continually multiplying gonidia. This growth is in an upward or out- 
ward direction away from the medulla, and strong mechanical pressure is 
exerted by the increasing tissue on the overlying cortical layers. Finally 
the soredia force their way through to the surface at definite points. The 
cortex is thrown back and forms a margin round the soralium, though shreds 
of epidermal tissue remain for a time mixed with the powdery granules. 

a. FORM AND OCCURRENCE OF SORALIA. The term " soralium " was 
first applied only to the highly developed soredial structures considered by 
Acharius to be secondary apothecia; it is now employed for any circum- 
scribed group of soredia. The soralia vary in size and form and in position, 
according to the species on which they occur; these characters are constant 
enough to be of considerable diagnostic value. Within the single genus 
Parmelia, they are to be found as small round dots sprinkled over the 
surface of P. dubia; as elongate furrows irregularly placed on P. sulcata; as 
pearly excrescences at or near the margins of P. perlata, and as swollen 
tubercles at the tips of the lobes of P.physodes (Fig. 83). Their development 
is strongly influenced and furthered by shade and moisture, and, given such 
conditions in excess, they may coalesce and cover large patches of the thallus 
with a powdery coating, though only in those species that would have borne 
soredia in fairly normal conditions. 

Soralia of definite form are of rather rare occurrence in crustaceous lichens, 

1 Wainio 1897, p. 32. 2 R e inke 1895, p. 380. 



STRUCTURES PECULIAR TO LICHENS 



145 



with the exception of the Pertusariaceae, where they are frequent, and some 
species of Lecanora and Placodium. They are known in only two hypo- 




Fig. 83. Parmelia physodes Ach. Thallus growing horizontally ; soredia on 
the ends of the lobes (S. H. , Photo.}. 



Xylographa spilomatica. 
Among squamulose thalli they are typical of some Cladoniae, and also of 
Lecidea (Psora) ostreata, where they are produced on the upper surface to- 
wards the apex of the squamule. 

b. POSITION OF SORALIFEROUS LOBES. According to observations'made 
by Bitter 1 , the occurrence of soralia on one lobe or another may depend to 
a considerable extent on the orientation of the thallus. He cites the varia- 
bility in habit of the familiar lichen, Parmelia physodes and its various forms, 
which grow on trees or on soil. In the horizontal thalli there is much less 
tendency to soredial formation, and the soredia that arise are generally 
confined to branching lobes on the older parts of the thallus. 

That type of growth is in marked contrast with the thallus obliged to 
take a vertical direction as on a tree. In such a case the lobes, growing 
downward from the point of origin, form soralia at their tips at an early 
stage (Fig. 84). The lateral lobes, and especially those that lie close to the 
substratum, are the next to become soraliate. Similar observations have 
been made on the soraliferous lobes of Cetraria pinastri. The cause is 
probably due to the greater excess of moisture draining downwards to the 
lower parts of the thallus. The lobes that bear the soralia are generally 



1 Bitter 



: 9 or. 



1 4 6 



MORPHOLOGY 



narrower than the others and are very frequently raised from contact with 
the substratum. They tend to grow out from the thallus in an upright 




Fig. 84. Parmelia physodes Ach. Thallus growing ver- 
tically ; soredia chiefly on the lobes directed downwards, 
reduced (M. P., Photo.'). 

direction and then to turn backwards at the tip, so that the opening of the 
soralium is directed downwards. Bitter says that the cause of this change 
in ^direction is not clear, though possibly on teleological reasoning it is of 
advantage that the opening of the soralium should be protected from direct 
rainfall. The opening lies midway between the upper and lower cortex, and 
the upper tissue in these capitate soralia continues to grow and to form an 
arched helmet or hood-covering which serves further to protect the soralium. 

Similar soralia are characteristic of Physcia Jiispida (Ph. stellaris subsp. 
tenella\ the apical helmet being a specially pronounced feature of that species, 
though, as Lesdain 1 has pointed out, the hooded structures are primarily 
the work of insects. In vertical substrata they occur on the lower lobes of 
the plant. 

Apical soralia are rare in fruticose lichens, but in an Alpine variety of 

1 Lesdain 1910. 



STRUCTURES PECULIAR TO LICHENS 147 

Ramalina minusctila they are formed at the tips of the fronds and are pro- 
tected by an extension of the upper cortical tissues. Another instance occurs 
in a Ramalina from New Granada referred by Nylander to R. calicaris var. 
farinacea: it presents a striking example of the helmet tip. 

c. DEEP-SEATED SORALIA. In the cases already described Schwendener 1 
and Nilson 2 held that the algae gave the first impulse to the formation of 
the soredia; but in the Pertusariaceae 3 , a family of crustaceous lichens, there 
has been evolved a type of endogenous soralium which originates with the 
medullary hyphae. In these, special hyphae rise from a weft of filaments 
situated just above the lowest layer of the thallus at the base of the medulla, 
the weft being distinguished from the surrounding tissue by staining blue 
with iodine. A loose strand of hyphae staining the usual yellow colour rises 
from the surface of the "blue" weft and, traversing the medullary tissue, 
surrounds the gonidia on the under side of the gonidial zone. The hyphae 
continue to' grow upward, pushing aside both the upper gonidial zone and 
the cortex, and carrying with them the algal cells first encountered. When 
the summit is reached, there follows a very active growth of both gonidia 
and hyphae. Each separate soredium so produced consists finally of five to 
ten algal cells surrounded by hyphae and measures 8/i to 13/14 in diameter. 
The cortex forms a well-defined wall or margin round the mass of soredia. 

A slightly different development is found in Lecanora tartarea, one of 
the "crottle" lichens, which has been placed by Darbishire in Pertu- 
sariaceae. The hyphae destined to form soredia also start from the weft of 
tissue at the base of the thallus, but they simply grow through the gonidial 
zone instead of pushing it aside. 

In his examination of Pertusariaceae Darbishire found that the apothecia 
also originated from a similar deeply seated blue-staining tissue, and he con- 
cluded that the soralia represented abortive apothecia and really corresponded 
to Acharius's "apothecia of the second order." His conclusion as to the 
homology of these two organs is disputed by Bitter 4 , who considers that 
the common point of origin is explained by the equal demand of the hyphae 
in both cases for special nutrition, and by the need of mechanical support 
at the base to enable the hyphae to reach the surface and to thrust back the 
cortex without deviating from their upward course through the tissues. 

C. DISPERSAL AND GERMINATION OF SOREDIA 

Soredia become free by the breaking down of the hyphal stalks at the 
septa or otherwise. They are widely dispersed by wind or water and soon 
make their appearance on any suitable exposed soil. Krabbe 5 has stated 

1 Schwendener 1860. 2 Nilson 1903. 3 Darbishire 1897. 4 Bitter 1901, p. 191. 

5 Krabbe 1891. 



148 MORPHOLOGY 

that, in many cases,- the loosely attached soredia coating some of the 
Cladonia podetia are of external origin, carried thither by the air-currents. 
Insects too aid in the work of dissemination: Darbishire 1 has told us how 
he watched small mites and other insects moving about over the soralia of 
Pertusaria amara and becoming completely powdered by the white granules. 

Darbishire 1 also gives an account of his experiments in the culture of 
soredia. He sowed them on poplar wood about the beginning of February 
in suitable conditions of moisture, etc. Long hyphal threads were at once 
produced from the filaments surrounding the gonidia, and gonidia that had 
become free were seen to divide repeatedly. Towards the end of August of 
the same year a few soredia had increased in size to about 450/11, in diameter, 
and were transferred to elm bark. By September they had further increased 
to a diameter of 520/1-, and the gonidia showed a tendency towards aggre- 
gation. No further differentiation or growth was noted. 

More success attended Tobler's 2 attempt to cultivate the soredia of 
Cladonia sp. He sowed them on soil kept suitably moist in a pot and after 
about nine months he obtained fully formed squamules, at first only an iso- 
lated one or two, but later a plentiful crop all over the surface of the soil. 
Tobler also adds that soredia taken from a Cladonia, that had been kept for 
about half a year in a dry room, grew when sown on a damp substratum. 
The algae however had suffered more or less from the prolonged desiccation, 
and some of them failed to develop. 

A suggestion has been made by Bitter 3 that a hybrid plant might result 
from the intermingling of soredia from the thallus of allied lichens. He 
proposed the theory to explain the great similarity between plants of Par- 
melia physodes and P. tubulosa growing in close proximity. There is no 
proof that such mingling of the fungal elements ever takes place. 

D. EVOLUTION OF SOREDIA 

Soredia have been compared to the gemmae of the Bryophytes and also 
to the slips and cuttings of the higher plants. There is a certain analogy 
between all forms of vegetative reproduction, but soredia are peculiar in 
that they include two dissimilar organisms. In the lichen kingdom there 
has been evolved this new form of propagation in order to secure the con- 
tinuance of the composite life, and, in a number of species, it has almost 
entirely superseded the somewhat uncertain method of spore germination 
inherited from the fungal ancestor, but which leaves more or less to chance 
the encounter with the algal symbiont. 

From a phylogenetic point of view we should regard the sorediate lichens 
as the more highly evolved, and those which have no soredia as phylo- 

3 Darbishire 1907. 2 Tobler 191 1 2 , n. 3 Bitter ipoi 2 . 



STRUCTURES PECULIAR TO LICHENS 149 

genetically young, though, as Lindau 1 has pointed out, soredia are all com- 
paratively recent. They probably did not appear until lichens had reached 
a more or less advanced stage of development, and, considering the poly- 
phyletic origin of lichens, they must have arisen at more than one point, 
and probably at first in circumstances where the formation of apothecia was 
hindered by prolonged conditions of shade and moisture. 

That soredia are ontogenetic in character, and not, as Nilson 2 has asserted, 
accidental products of excessively moist conditions is further proved by the 
non-sorediate character of those species oforustaceous lichens belonging to 
Lecanora, Verrucaria, etc. that are frequently immersed in water. Bitter 3 
found that the soredia occurring on Peltigera spuria were not formed on the 
lobes which were more constantly moist, nor at the edges where the cortex 
was thinnest: they always emerged on young parts of the thallus a short 
way back from the edge. 

Bitter 3 points out that in extremely unfavourable circumstances in the 
polluted atmosphere near towns, or in persistent shade lichens, that would 
otherwise form a normal thallus, remain in a backward sorediose state. He 
considers, however, that many of these formless crusts are autonomous growths 
with specific morphological and chemical peculiarities. They hold these 
outposts of lichen vegetation and are not found growing in any other localities. 
The proof would be to transport them to more favourable conditions, and 
watch development. 

4. ISIDIA 
A. FORM AND STRUCTURE OF ISIDIA 

Many lichens are rough and scabrous on the surface, with minute simple 
or divided coral-like outgrowths of the same texture as the underlying thallus, 
though sometimes they are darker in colour as in Evernia furfuracea. They 
always contain gonidia and are covered by a cortex continuous with that 
of the thallus. 

This very marked condition was considered by Acharius 4 as of generic 
importance and the genus, Isidium, was established byhim, with the diagnostic 
characters: "branchlets produced on the surface, or coralloid, simple and 
branched." In the genus were included the more densely isidioid states of 
various crustaceous species such as Isidium corallinum and /. Westringii, 
both of which are varieties of Pertusariae. Fries 5 , with his accustomed insight, 
recognized them as only growth forms. The genus was however still accepted 
in English Floras 6 as late as 1833, though we find it dropped by Taylor 7 in 
the Flora Hibernica a few years later. 

1 Lindau 1895. 2 Nilson 1903. 3 Bitter 1904. 4 Acharius 1798, pp. 2, 87. 

5 Fries 1825. 6 Hooker 1833. 7 Taylor 1836. 



MORPHOLOGY 




The development of the isidial outgrowth has been described by Rosen- 
dahl 1 in several species of Parmelia. In one of them, P. papulosa, which has 
a cortical layer one cell thick, the isidium begins as a small swelling or wart 
on the upper surface of the thallus. At that stage the cells of the cortex 
have already lost their normal arrangement and show irregular division. 
They divide still further, as gonidia and hyphae push their way up. The 
full-grown isidia in this species are cylindrical or clavate, simple or branched. 
They are peculiar in that they bear laterally 
here and there minute rhizoids, a development 
not recorded in any other isidia. The inner 
tissue accords with that of the normal thallus 
and there is a clearly marked cortex, gonidial 
zone and pith. A somewhat analogous develop- 
ment takes place in the isidia of Parmelia pro- 
boscidea; in that lichen they are mostly pro- 
longed into a dark-coloured cilium. 

In Parmelia scortea the cortex is several 
cells thick, and the outermost rows are com- 
pressed and dead in the older parts of the 
thallus; but here also the first appearance of 
the isidium is in the form of a minute wart. 
The lower layers (4 to 6) of living cortical cells 
divide actively; the gonidia also share in the 
new growth, and the protuberance thus formed 
pushes off the outer dead cortex and emerges 
: 60 (after as an isidium (Fig. 85). They are always rather 
stouter in form than those of P. papulosa and 
may be simple or branched. The gonidia in this case do not form a 
definite zone, but are scattered through the pith of the isidium. 

Here also should be included the coralloid branching isidia that adorn 
the upper surface and margins of the thallus of Umbilicaria pusttilata. 
They begin as small tufts of somewhat cylindrical bodies, but they some- 
times broaden out to almost leafy expansions with crisp edges. Most 
frequently they are situated on the bulging pustules where intercalary 
growth is active. Owing to their continued development on these areas, 
the tissue becomes slack, and the centre of the isidial tuft may fall out, 
leaving a hole in the thallus which becomes still more open by the tension 
of thalline expansion. New isidia sprout from the edges of the wound and 
the process may again be repeated. It has been asserted that these structures 
are only formed on injured parts of the thallus something like gall- 
formations but Bitter 2 has proved that the wound is first occasioned by 
the isidial growth weakening the thallus. 

1 Rosendahl 1907. 2 Bitter 1899. 



Fig. 85. Vertical section of isidia of 
Parmelia scortea Ach. A, early 



stage; B. later stage, 
Rosendahl). 



STRUCTURES PECULIAR TO LICHENS 151 



B. ORIGIN AND FUNCTION OF ISIDIA 

Nilson 1 (later Kajanus 2 ) insists that isidia and soredia are both products 
of excessive moisture. He argues that lichen species, in the course of their 
development, have become adapted to a certain degree of humidity, and, if 
the optimum is passed, the new conditions entail a change in the growth 
of the plant. The gonidia are stimulated to increased growth, and the 
mechanical pressure exerted by the multiplying cells either results in the 
emergence of isidial structures where the cortex is unbroken, or, if the 
cortex is weaker and easily bursts, in the formation of soralia. 

This view can hardly be accepted ; isidia as well as soredia are typical 
of certain species and are produced regularly and normally in ordinary 
conditions; both of them are often present on the same thallus. It is not 
denied, however, that their development in certain instances is furthered 
by increased shade or moisture. In Evernia furfuracea isidia are more 
freely produced on the older more shaded parts of the thallus. Zopf 3 has 
described such an instance in Evernia olivetorina (E. furfuracea)^ which 
grew in the high Alps on pine trees, and which was much more isidiose 
when it grew on the outer ends of the branches, where dew, rain or snow 
had more direct influence. He 4 quotes other examples occurring in forms 
of E. furfuracea which grew on the branches of pines, larches, etc. in a damp 
locality in S. Tyrol. The thalli hung in great abundance on each side of 
the branches, and were invariably more isidiose near the tips, because 
evidently the water or snow trickled down and was retained longer there 
than at the base. 

Bitter 5 has given a striking instance of shade influence in Umbilicaria. 
He found that some boulders on which the lichen grew freely had become 
covered over with fallen pine needles. The result was at first an enormous 
increase of the coralline isidia, though finally the lichen was killed by the 
want of light. 

Isidia are primarily of service to the plant in increasing the assimilating 
surface. Occasionally they grow out into new thallus lobes. The more 
slender are easily rubbed off, and, when scattered, become efficient organs 
of propagation. This view of their function is emphasized by Bitter who 
points out that both in Evernia furfuracea and in Umbilicaria pustulata 
other organs of reproduction are rare or absent. Zopf 3 found new plants 
of Evernia furfuracea beginning to grow on the trunk of a tree lower 
down than an old isidiose specimen. They had developed from isidia which 
had been detached and washed down by rain. 

1 Nilson 1903. a Kajanus (Nilson) 1911. 3 Zopf 1903. 4 Zopf 19052. 

5 Bitter 1899. 



1 52 MORPHOLOGY 

VI. HYMENOLICHENS 
A. SUPPOSED AFFINITY WITH OTHER PLANTS 

Lichens in which the fungal elements belong to the Hymenomycetes 
are confined to three tropical genera. They are associated with blue-green 
algae and are most nearly related to the Thelephoraceae among fungi. The 
spores are borne, as in that family, on basidia. 

The best known Hymenolichen, Cora Pavonia (Fig. 86), was discovered 
by Swartz 1 during his travels in the W. Indies (1785-87) growing on trees 




Fig. 86. Cora Pavonia Fr. (after Mattirolo). 

in the mountains of Jamaica, and the new plant was recorded by him as 
Ulva montana. Gmelin 2 also included it in Ulva in close association with 
Ulva (Padind) Pavonia, but that classification was shortly after disputed by 
Woodward 3 who thought its affinity was more nearly with the fungi and 
suggested that it should be made the type of a new genus near to Boletus 
(Polystictus} versicolor. Fries 4 in due time made the new genus Cora, though 
he included it among algae; finally N.ylander 5 established the lichenoid 
character of the thallus and transferred it to the Lecanorei. 

It was made the subject of more exact investigation by Mattirolo 6 who 

1 Swartz 1788. 2 Gmelin 1791. 3 Woodward 1797. 4 Fries 1825. 

5 Nylander 1855. 6 Mattirolo 1881. 



HYMENOLICHENS 



153 



recognized its affinity with Thelephora, a genus of Hymenomycetes. Later 
Johow 1 went to the West Indies and studied the Hymenolichens in their 
native home. The genera and species described by Johow have been 
reduced to Cora and Dictyonema ; a new genus Corella has since been added 
by Wainio 2 . 

Johow found that Cora grew on the mountains usually from 1000 to 
2000 ft. above sea-level. As it requires for its 
development a cool damp climate with strong 
though indirect illumination, it is found 
neither 1 in sunny situations nor in the depths 
of dark woods. It grows most freely in diffuse 
light, on the lower trunks and branches of 
trees in open situations, but high up on the 
stem where the vegetation is more dense. 
It stands out from the tree like a small thin 
bracket fungus, one specimen placed above 
another, with a dimidiate growth similar to 
that of Polystictus versicolor. Both surfaces 
are marked by concentric zones which give 
it an appearance somewhat like Padina Pa- 
vonia. These zones indicate unequal inter- 
calary growth both above and below. The 
whole plant is blue-green when wet, greyish- 
white when dry, and of a thin membranaceous 
consistency. 

B. STRUCTURE OF THALLUS 

There is no proper cortex in any of the 
genera, but in Cora there is a fastigiate 
branching of the hyphae in parallel lines 
towards the upper surface; just at the outside 
they turn and lie in a horizontal direction, 
and, as the branching becomes more profuse, 
a rather compact cover is formed. The goni- 
dia, which consist of blue-green Chroococcus 
cells, lie at the base of the upward branches 
and they are surrounded with thin-walled short-celled hyphae closely inter- 
woven into a kind of cellular tissue. The medulla of loose hyphae passes 
over to the lower cortex, also of more or less loose filaments. The outermost 
cells of the latter very frequently grow out into short jagged or crenate 

processes (Fig. 87). 

1 Johow 1884. 2 Wainio 1890. 




Fig. 87. Cora Pavonia Fr. Vertical 
section of thallus. a, upper cortex ; 
b, gonidial layer; t, medulla and 
lower cortex of crenate cells; d, tuft 
of fertile hyphae. x 160. e, basidia 
and spores x 1000 (after Johow). 



154 MORPHOLOGY 

In Corella, the mature lichen is squamulose or consists of small lobes; in 
Dictyonema there is a rather flat dimidiate expansion; in both the alga is 
Scyt0nema,thetrichomes of which largely retain their form and are surrounded 
by parallel growths of branching hyphae. The whole tissue is loose and 
spongy. 

Corella spreads over soil on a white hypothallus without rhizinae. In 
the other two genera which live on soil, or more frequently on trees, there 
is a rather extensive formation of hold-fast tissue. When the dimidiate 
thallus grows on a rough bark, rhizoidal strands of hyphae travel over it 
and penetrate between the cracks; if the bark is smooth, there is a more 
continuous weft of hyphae. In both cases a spongy cushion of filamentous 
tissue develops at the base of the lichen between the tree and the bracket 
thallus. There is also in both genera an encrusting form which Johow 
regarded as representing a distinct genus Laudatea, but which Moller found 
to be merely a growth stage. Moller 1 judged from that and from other 
characteristics that the same fungus enters into the composition of both 
Cora and Dictyonema and that only the algal constituents are different. 

C. SPORIFEROUS TISSUES 

As in Hymenomycetes, the spores of Hymenolichens are exogenous, 
and are borne at the tips of basidia which in these lichens are produced on 
the under surface of the thallus. In Cora the fertile filaments may form a 
continuous series of basidia over the surface, but generally they grow out 
in separate though crowded tufts. As these tufts broaden outwards, they 
tend to unite at the free edges, and may finally present a continuous 
hymenial layer. Each basidium bears four sterigmata and spores (Fig. 87 e}\ 
paraphyses exactly similar to the basidia are abundant in the hymenium. 
In Dictyonema the hymenium is less regular, but otherwise it resembles that 
of Cora. No hymenium has as yet been observed in Corella; it includes, so 
far as known, one species, C. brasiliensis , which spreads over soil or rocks. 

1 Moller 1893. 



CHAPTER IV 

REPRODUCTION 
I. REPRODUCTION BY ASCOSPORES 

A. HISTORICAL SURVEY 

THE earliest observations as to the propagation of lichens were made by 
Malpighi 1 who recorded the presence of soredia on the lichen plant and 
noted their function as reproductive bodies. He was followed after a con- 
siderable interval by Tournefort 2 who placed lichens in a class apart owing 
to the form of the fruit: "This fruit," he writes, "is a species of bason or 
cup which seems to take the place of seeds in these kinds of plants." He 
figures Ramalina fraxinea and Physcia ciliaris, both well fruited specimens, 
and he represents the " minute dust " contained in the fruits as subrotund 
in form. The spores of Physcia ciliaris are of a large size and dark in colour 
and were undoubtedly seen by Tournefort. Morison 3 , in his History of 
Oxford Plants, published very shortly after, dismissed Tournefort's "seeds" 
as being too minute to be of any practical interest. 

Micheli 4 , with truer scientific insight, made the fruiting organs the subject 
of special study. He decided that the apothecia were floral receptacles, 
receptacula florum, and that the spores were the " flowers " of the lichen. He 
has figured them in a vertical series in situ, in a section of the disc of Solorina 
saccata 6 and also in a species of Pertusaria 5 , in both of which plants the 
ascospores are unusually large. He adds that he had not so far seen the 
" semina." 

Micheli's views were not shared by his immediate successors. Dillenius 6 
scarcely believed that the spores could be " flowers " and, in any case, he 
concluded that they were too minute to be of any real significance in the 
life of the plant. 

Linnaeus 7 , and after him Necker 8 , Scopoli 9 and others describe the apo- 
thecia as the male, the soredia as the female organs of lichens. These old 
time botanists worked with very low powers of magnification, and easily went 
astray in the interpretation of imperfectly seen phenomena. 

Koelreuter 10 , a Professor of Natural History in Carlsruhe, who pub- 
lished a work on The discovered Secret of Cryptogams, next hazarded the 
opinion that the seeds of lichens originated from the substance of the pith, 
and that the overlying cortical layer supplied the fertilizing sap. Hoffmann 11 

1 Malpighi 1686. 2 Tournefort 1694. 3 Morison 1699. 4 Micheli 1729. 

3 Micheli, Pis. 52 and 56. B Dillenius 1741. 7 Linnaeus 1737. 8 Necker 1771, p. 257. 

" Scopoli 1772. 10 Koelreuter 1777. u Hoffmann 1784. 



156 REPRODUCTION 

devoted a great deal of attention to lichen fructification and he also thought 
that fertilization must take place within the tissue of the lichens. He 
regarded the soredia as the true seeds, while allowing that a second series 
of seeds might be contained in the scutellae (apothecia). 

A distinct advance was made by Hedwig 1 , a Professor of Botany in 
Leipzig, towards the end of the eighteenth century. He followed Tourne- 
fort in selecting Physcia ciliaris for research, and in that plant he describes 
and figures not only the apothecia with the dark-coloured septate spores, 
but also the pycnidia or spermogonia which he regarded as male organs. 
The soredia, typically represented and figured by him on Parmelia physodes, 
he judged to be " male flowers of a different type." 

Acharius 2 did not add much to the knowledge of reproduction in lichens, 
though he takes ample note of the various fruiting structures for which he 
invented the terms apothecia, perithecia and soredia. Under still another 
term gongyli he included not only spores, but the spore guttulae as well as 
the gonidia or cells forming the soredia. 

Hornschuch 3 of Greifswald described the propagation of the lower lichens 
as being solely by means of a germinating " powder " ; the more highly or- 
ganized forms were provided with receptacles or apothecia containing spores 
which he considered as analogous to flowers rather than to fruits. The im- 
portant contributions to Lichenology of Wallroth 4 and Meyer 5 close this 
period of uncertainty: the former deals almost exclusively with the form 
and character of the vegetative thallus and the function of the " reproductive 
gonidia." Meyer, a less prolix writer, very clearly states that the method of 
reproduction is twofold: by spores produced in fruits, or by the germinating 
granules of the soredia. 



B. FORMS OF REPRODUCTIVE ORGANS 

From the time of Tournefort, considerable attention had been given to 
the various forms of scutellae, tuberculae, etc., as characters of diagnostic 
importance. Sprengel 6 grouped these bodies finally into nine different types 
with appropriate names which have now been mostly superseded by the 
comprehensive terms, apothecia and perithecia. A general classification on 
the lines of fruit development was established by Luyken 7 , who, following 
Persoon's 8 classification of fungi, and thus recognizing their affinity, summed 
up all known lichens as Gymnocarpeae with open fruits, and Angiocarpeae 
with closed fruits. 

a. APOTHECIA. As in discomycetous fungi, the lichen apothecium is 
in the form of an open concave or convex disc, but generally of rather small 

1 Hedwig 1784. 2 Acharius 1810. 3 Hornschuch 1821. 4 Wallroth 1825. 

6 Meyer 1825. 6 Sprengel 1804. ? Luyken 1809. Pe rsoon 1801. 



REPRODUCTIVE ORGANS 



157 



size, rarely more than I cm. in diameter (Fig. 88); there is no development 
in lichen fruits equal to the cup-like ascomata of the larger Pezizae. In 




Fig. 88. Lecanora subfusca Ach. A, thallus and apothecia x 3 ; 
B, vertical section of apothecium. a, hymenium; b, hypo- 
thecium; c, thalline margin or amphithecium ; of, gonidia. 
x 60 (after Reinke). 

most cases the lichen apothecium retains its vitality as a spore-bearing 
organ for a considerable period, sometimes for several years, and it is 
strengthened and protected by one or more external margins of sterile 
tissue. Immediately surrounding the fertile disc there is a compact wall of 
interwoven hyphae. In some of the shorter-lived soft fruits, as in Biatora, 
this hyphal margin may be thin, and may gradually be pushed aside as the 
disc develops and becomes convex, but generally it forms a prominent rim 
round the disc and may be tough or even horny, and often hard and car- 
bonaceous. This wall, which is present, to some extent, in nearly all lichens, 
is described as the "proper margin." A second "thalline margin" containing 
gonidia is present in many genera 1 : it is a structure peculiar to the lichen 
apothecium and forms the amphithecium. 

At the base of the apothecium there is a weft of light- or dark-coloured 
hyphae called the hypothecium> which is continued up and round the sides 
as the parathecium merging into the "proper margin." It forms the lining 
of a cup-shaped hollow which is filled by the paraphyses, which are upright 
closely packed thread-like hyphae, and by the'spore-containing asci or thecae, 
these together constituting the thecium or hymenium. The paraphyses 
are very numerous as compared with the asci ; they are simple or branched, 

1 See also p. 166. 



I 5 8 REPRODUCTION 

frequently septate, especially towards the apex, and mostly slender, varying in 
width from 1-4/4, though Hue describes paraphyses in Aspicilia atroviolacea 
as 8-12/u, thick. They may be thread-like throughout their length, or they 
may widen towards the tips which are not infrequently coloured. Small 
apical cells are often abstricted and lie loose on the epithecium, giving at 
times a pruinose or powdered character to the disc. In some genera there 
is a profuse branching of the paraphyses to form a dense protective epithecium 
over the surface of the hymenium as in the genus Arthonia. 

The apothecia may be sessile and closely adnate to or even sunk in the 
thallus, or they may be shortly stalked. The thalline margin shares generally 
the characters of the thallus; the disc is mostly of a firm consistency and is 
light or dark in colour according to genus or species ; most frequently it is 
some shade of brown. Marginate apothecia, i.e. those with a thalline margin, 
are often referred to as "lecanorine," that being a distinctive feature of 
the genus Lecanora. In the immarginate series, with a proper margin 
only, the texture may be soft and waxy, termed "biatorine" as in Biatora; 
or hard and carbonaceous as in the genus Lecidea, and is then described as 
"lecideine." 

In the subseries Graphidineae, the apothecium has the form of a very 
flat, roundish or irregular body entirely without 
a margin, called an "ardella" as in Arthonia; 
or more generally it is an elongate narrow 
"lirella," in which the disc is a mere slit 
"> between two dark-coloured proper margins. 
The hypothecium of the lirellae is sometimes 
much reduced and in that case the hymenium 
rests directly on a thin layer above the thalline 
tissue as in Graphis elegans (Fig. 89). 

Lichen fruits require abundant light, and 
plants growing in the shade are mostly sterile. 
B Naturally, therefore, the reproductive bodies 

Fig. 89. Graphis elegans Ach. A,. , f , , , .,, 

thallus and lirellae; B, vertical are lo be found on the best illuminated parts 

section of furrowed lirella. x ca. of the thallus. In crustaceous and in most 

foliose forms, they are variously situated on 

the upper surface, wherever the light falls most directly. In the genera 
Nephromium and Nephromopsis, on the contrary, they arise on the under sur- 
face, though at the extreme margin, but as the fertile lobes eventually turn 
upwards the apothecia as they mature become fully exposed. In shrubby 
or fruticose lichens their position is lateral on the fronds, or more frequently 
at or near the tips. 

b. PERITHECIA. The small closed perithecium is characteristic of the 
Pyrenocarpeae which correspond with the Pyrenomycetes among fungi. It 





REPRODUCTIVE ORGANS 159 

is partially or entirely immersed in the thallus or in the substratum on 
which the lichen grows, and is either a globose or conical body wholly 
surrounded by a hyphal wall, when it is de- 
scribed as "entire" (Fig. 90), or it is somewhat 
hemispherical in form and the outer wall is 
developed only on the upper exposed part: 
a type of perithecium usually designated by 
the term "dimidiate." As the perithecial wall 
gives sufficient protection to the asci, the 
paraphyses are of less importance and are 
frequently very sparingly produced, or they 
may even be dissolved and used up at an early 
stage. The thallus of the Pyrenocarpeae is 
often extremelyreduced, and the perithecia are F 'g- 9- A . e ^'^ perithecium of 

J l Poiina ohvacea A. L.Sm. x ca.4o; 

then the only Visible portion of the lichen. B, dimidiate perithecium of Acro- 

A few lichens among Graphidineae and 

Pyrenocarpeae grow in a united body generally looked on as a stroma; 
but Wainio 1 has demonstrated that as the fruiting bodies give rise to this 
structure by agglomeration by the cohesion of their margins it can only 
be regarded as a pseudostroma. Two British genera of Pyrenolichens, 
Mycoporum and Mycoporellum, exhibit this pseudo-stromatoid formation. 



C. DEVELOPMENT OF REPRODUCTIVE ORGANS 

As most known lichens belong to the Ascolichens, the study of develop- 
ment has been concentrated on that group. Tulasne 2 was the first to make 
a microscopic study of lichen tissues and he described in considerable detail 
the general anatomical structure of apothecia and perithecia. Later, Fuisting :i 
traced the development of a number of perithecia through their different 
stages of growth, but his most interesting discovery was made in Lecidea 
fumosa, a crustaceous Discolichen with an areolate thallus in which the 
apothecia are seated on the fungal hyphae between the areolae. In the very 
early stages represented by a complex of slender hyphae, he observed an 
unbranched septate filament with short cuboid cells, richer in contents than 
the surrounding filaments and somewhat similar to the structure known to 
mycologists as "Woronin's hypha," which is an ascogonial structure. These 
specialized cells disappeared as the hymenium began to form. 

1 Wainio 1890. Tulasne 1852. 3 Fuisting 1868. 



i6o 



REPRODUCTION 



i. DISCOLICHENS 

a. CARPOGONIA OF GELATINOUS LICHENS. Stahl's 1 work on various 
Collemaceae followed on the same lines as that of Fuisting. The first species 
selected by him for examination, Collema (Leptogium) microphyllum, is a 
gelatinous lichen which grows on old trunks of poplars and willows. It has 
a small olive-green thallus which, in autumn, is crowded with apothecia; 
the spermogones or pycnidia appear as minute reddish points on the edge of 
the thallus. Within the thallus, and midway between the upper and lower 
surface, there arises, as a branch from a vegetative hypha, a many-septate 
filament coiled in spiral form at the base, with the free end growing upwards 
and projecting a short distance above the surface and occasionally forked 
(Fig. 91). The tip-cell is slightly swollen and covered with a mucilaginous 

.6 




Fig. 91. Collema microphyllum Ach. Vertical section of 
thallus. a, carpogonium ; b, trichogyne. x 350 (after 
Stahl). 

coat continuous with the mucilage of the thallus. The whole structure, 
characterized by the larger size and by the richer contents of its cells, was 
regarded by Stahl as a carpogonium, the coiled base representing the asco- 
gonium, the upright hypha functioning as the receptive organ or trichogyne, 
comparable to that of the Florideae. The spermatia, which mature at this 
early stage of carpogonial development, are expelled from a neighbouring 
spermogonium on the addition of moisture and easily reach the protruding 
trichogyne. They adhere to the mucilaginous wall of the end-cell, and, in 
two or three instances, Stahl found that copulation had taken place. As the 
affixed spermatium was empty, he concluded that the contents had passed 
over into the trichogyne, and that the nucleus had travelled down to the 
ascogonium. Certain degenerative changes that followed seemed to confirm 

1 Stahl 1877. 



REPRODUCTION IN DISCOLICHENS 



161 




the view that there had been fertilization: the cells of the trichogynej had 
lost their turgidity and at the same time the cross-walls had swollen con- 
siderably and stood out like knots in the 
hypha (Fig. 92). The ascogonial cells had 
also increased not only in size but in number 
by intercalary division, so that the spiral 
arrangement became obscured. Ascogenous 
hyphae arose from the ascogonial cells, and 
asci cut off by a basal septum were finally 
formed from these hyphae. Lateral branches 
from below the septum also formed asci. 

Stahl's observations were repeated and 
extended by Borzi 1 on another of the Colle- 
maceae, Collema nigrescens. In that plant the 
foliaceous thallus is of thin texture and has 
a distinct cellular cortex. The carpogonia 
were found at varying depths near to the cor- 
tical region; the ascogonium, of two and a 
half to four spirals, consisted often to fifteen 
cells with very thin walls, the trichogyne of 
five to ten cells, the terminal cell projecting 
above the thallus. Borzi also found spermatia 
fused with the tip-cell. 

A further important contribution was made by Baur" in his study of 
Collema crispum*. There occur in nature two forms of this lichen, one of 
them crowded with apothecia and spermogonia, the other with a more 
luxuriant thallus, but with few apothecia and no spermogonia. On the latter 
almost sterile form Baur found in spring and again in autumn immense 
numbers of carpogonia about one thousand in a medium sized thallus 
which nearly all gradually lost the characteristics of reproductive organs, 
and, anastomising with other hyphae, became part of the vegetative system. 
In a few cases in which, presumably, a spermatium had fused with a tricho- 
gyne, very large apothecia had developed. 

As the first-mentioned form was always crowded with apothecia in every 
stage of development, as well as with carpogonia and spermogonia, it seemed 
natural to conclude that the difference was entirely due to the presence or 
absence of spermatia in sufficient numbers to ensure fertilization. The 
period during which copulation is possible passes very rapidly, though 
subsequent development is slow, occupying about half-a-year from the time 
of fertilization to the formation of the first ascus. 

1 Borzi 1878. 2 Baur 1898. 

3 Fiinfstiick (1902) suggests that the lichen worked at by Baur is Collema cheileuni Ach. 



Fig. 92. Collema microphyllum Ach. 
Carpogonium and trichogyne after 
copulation x 500 (after Stahl). 



I62 REPRODUCTION 

Baur confirmed Stahl's observations on the various developmental 
changes. In several instances he found a spermatium fused with the tricho- 
gyne, though he could not see continuity between the lumina of the fusing 
cells. After copulation with the spermatium the trichogyne nucleus, which 
occupied the lower third of the terminal cell, had disappeared, and the 
plasma contents had acquired a deeper tint; the other trichogyne cells, 
which had also lost their nuclei, were partly collapsed owing to the pressure 
of the surrounding tissue, and openings were plainly visible through some 
of the swollen septa, especially of the lower cells. In addition the ascogonial 
cells, all of which were uninucleate, had increased in number by intercalary 
division. Plasma connections were opened from cell to cell, but only in the 
primary septa, the later formed cell-membranes being continuous. Asco- 
genous hyphae had branched out from the ascogonium as a series of 
uninucleate cell rows from which the asci finally arose. 

Baur's interpretation was that the first cell of the ascogonium reached 
by the male nucleus after its passage down through the cells of the trichogyne 
represented the egg-cell, and that, after fusion, the resultant nucleus divided, 
and a daughter nucleus passed on to the other auxiliary-cells. No male 
nucleus nor fusion of nuclei was, however, observed by him, and his deduc- 
tions rest on conjecture. 

Krabbe 1 and after him Maule 2 found in Collema pulposum reproductive 
organs similar to those described by Stahl, but in a recent paper on an 
American form of that species a peculiar condition has been described 
by Freda Bachmann 3 . She 4 found that the spermatia originated, not in 
spermogonia, but as groups of cells budded off from vegetative hyphae 
within the tissue of the lichen and occupying the same position as spermo- 
gonia, i.e. the region close below the upper surface. The trichogynes, therefore, 
never emerged into the open, but travelled towards these internal spermatia, 
and fusion with them was effected. The changes that afterwards took place 
in the carpogonial cells were similar to those that had been recognized by 
Stahl and Baur as consequent on fertilization. 

Additional cytological details have been published in a subsequent 
paper 5 : after fusion with the spermatium the terminal cell of the trichogyne 
collapsed, its nucleus became disintegrated and the cross septa of the lower 
trichogyne cells became perforated, these perforations being closed again at 
a later stage by a gelatinous plug. The nuclear history is more doubtful : 
the disappearance of the nuclei from the spermatium and from the terminal 
cell of the trichogyne was noted; two nuclei were seen to be present in the 
penultimate cell, and these the author interpreted as division products of the 

1 Krabbe 1883. 2 Maule 1891. 3 F. Bachmann 1912. 

4 This species of Collema has been described as Collemodes Bachmanniantim by Bruce Fink 1918. 

5 F. Bachmann 1913. 



REPRODUCTION IN DISCOLICHENS 163 

original cell nucleus. In the same cell, lying close against the lower septum 
and partly within the opening, there was a mass of chromatin material which 
might be the male nucleus migrating downwards. The next point of interest 
was observed in the twelfth cell from the tip in which there were two nuclei, 
a larger and a smaller, the latter judged to be the male cell, the small size 
being due to probable division of the spermatium nucleus either before or 
after leaving the spermatium. It is stated however that the spermatium 
was always uninucleate. Meanwhile the cells of the ascogonium had 
increased in size, the perforations of the septa between the cells became 
more evident, and their nuclei persisted. In one cell at this stage two nuclei 
were present, one of the two presumably a male nucleus; no fusion of nuclei 
was observed in the ascogonial cells. Later the cross walls between the 
cells were seen to have disappeared more completely and migration of 
nuclei had taken place, so that some of the cells appeared to be empty while 
others were multinucleate. Considerable multiplication of the nuclei occurred 
before the ascogenous hyphae were formed : twelve nuclei were observed in 
a part of the ascogonium which was just beginning to give off a branch. 
Several branches might arise from one cell, and their cells were either uni- 
or binucleate, the nuclei being larger than those of the vegetative hyphae. 
The formation of the asci was not distinctly seen, but young binucleate 
asci were not uncommon. The fusion of the two nuclei was followed by 
the enlargement of the ascus and the subsequent nuclear division for spore 
formation. In the first heterotypic division twelve chromosomes, double the 
number observed in the vegetative nucleus, were counted on the equatorial 
plate. In the third division they were reduced to the normal number of six, 
from which F. Bachmann concludes that a twofold fusion must have taken 
place in the ascogonium and again in the ascus. 

Spiral or coiled ascogonia were observed by Wainio 1 in the gelatinous 
crustaceous genus Pyrenopsis, but the trichogynes did not reach the surface. 
In Lichina-, a maritime gelatinous lichen where the carpogonia occur in 
groups, trichogynes have not been demonstrated. 

A peculiarity of some gelatinous lichens noted by Stahl 3 and others in 
species of Pkysma, and by Forssell 4 in Pyrenopsis and Psorotichia, is the 
development of carpogonia at the base of, and within the perithecial walls 
of old spermogonia. No special significance is however attached to this 
phenomenon, and it is interesting to note that a similar growth was observed 
by Zukal 5 in a pyrenomycetous fungus, Pleospora collematum, a harmless 
parasite on PJiysma compactum and other Collemaceae. The structures in- 
vaded were true pycnidia of the fungus as the minute spores were seen to 
germinate. A " Woronin's hypha " at the base of several of these pycnidia 
developed asci which pushed up among the spent sporophores. 

1 Wainio i. 1890. 2 Wolff 1905. 3 Stahl 1877. 4 Forssell 1885'-. a Zukal 1887, p. 42. 

II 2 



1 64 



REPRODUCTION 



b. CARPOGONIA or NON-GELATINOUS LICHENS. The soft loose tissue 
of the gelatinous lichens is more favourable for the minute study of apo- 
thecial development than the closely interwoven hyphae of non-gelatinous 
forms, but Borzi 1 had already extended the study to species of Parmelia, 
Anaptychia, Sticta, Ricasolia and Lecanora, and in all of them he succeeded 
in establishing the presence of ascogonia and trichogynes. After him a 
constant succession of students have worked at the problem of reproduction 
in lichens. 

Lindau 2 published results of the examination of a considerable series of 
lichens. In Anaptychia (Physcia) ciliaris, Physcia stellaris, Ph. pulverulenta, 
Ramalina fraxinea, Placodium (Lecanora) saxicolum, Lecanora subfusca and 
Lecidea enteroleuca he demonstrated the presence of ascogonia with tricho- 
gynes. In Parmelia tiliacea and in Xanthoria parietina he found ascogonia 
but failed to see trichogynes. In none of the species examined by him did 
he observe any fusion between the trichogyne and a spermatium. 

In Anaptychia ciliaris he was able to pick out extremely early stages by 
staining with a solution of chlor-zinc-iodine. Maule 3 applied the same test 
to Physcia pulverulenta, but found that to be successful the reaction required 
some time. Certain cells of the hyphae mostly 
terminal cells in the lower area of the gonidial 
zone and even occasionally in the pith (according 
to Lindau) coloured a deep brown, while the 
ordinary thalline hyphae were tinted yellow. 
He assumed that these were initial ascogonial 
cells on account of the richer plasma contents, 
and also because of the somewhat larger size of 
the cells. In the same region of the thallus 
young carpogonia were observed as outgrowths 
from vegetative hyphae, though the trichogynes 
had not yet reached the surface. 

At a more advanced stage the carpogonia 
were seen to be embedded in the gonidial zone 
and occurred in groups. The cells of the asco- 
gonium, easily recognized by the darker stain, 
were short and stout, measuring about 6-8 /j, in 
length and 4*4 p, in width. They were arranged 
in somewhat indistinct spirals; but the crowding 
of the groups resulted in a confused intermingling of the various generative 
filaments. The trichogynes composed of longer narrower cells rose above 
the hyphae of the cortex; they also stained a deep brown and the projecting 
cell was always thin-walled. Lindau frequently observed spermatia very 
1 Borzi 1878. 2 Lindau 1888. 3 Maule 1891. 




Fig. 93. Pkyscia pulverulenta Nyl. 
Vertical section of thallus and 
carpogonium before fertilization. 
a, outer cortex; b, inner cortex; 
c, gonidial 1 ayer ; d, medulla, 
x ca. 540 (after Darbishire). 



REPRODUCTION IN DISCOLICHENS 



165 



firmly attached to the trichogyne cell but without any plasma connection 
between the two. The changes in the trichogyne described by Stahl and 
Baur in Collemaceae were not seen in Anaptychia\ the peculiar swelling of 
the septa seems to be a phenomenon confined to gelatinous lichens. During 
the trichogyne stage in this lichen the vegetative hyphae from the medulla 
grow up and surround the young carpogonia, and, at the same time, very 
slender hyphae begin to branch upwards to form the paraphyses. Darbi- 
shire's 1 examination of Physcia pulvernlenta demonstrated the presence of 
the coiled ascogonium and the trichogyne in that species (Fig. 93). 

Baur 1 has also given the results of an examination of Anaptychia. He 
frequently observed copulation between the spermatium and the tip- of the 
trichogyne, but not any passage of nucleus or contents. After copulation 
the ascogonial cells increased in size and became irregular in form, and 
open communication was established between them (Fig. 94). There was 
no increase in their number by intercalary division as in Collema. After 




Fig. 94. Physcia {Anaptychia) ciliaris DC. Vertical 
section of developing ascogonium. a, paraphyses ; 
b, ascogonial hyphae; c, ascogonial cells, x 800 (after 
Baur). 

producing ascogenous hyphae the cells were seen to have lost their contents 
and then to have gradually disappeared. The fertile hyphae, which now 
took a blue colouration with chlor-zinc-iodine, gradually spread out and 

1 Darbishire 1900. 2 Baur 1904. 



1 66 REPRODUCTION 

formed a wide-stretching hymenium. Several carpogonia took part in the 
formation of one apothecium. 

The tissue below the ascogonium meanwhile developed vigorously, form- 
ing a weft of encircling hyphae, while the upper branches grew vertically to- 
wards the cortex. Gonidia in contact with the developing fruit also increased, 
and, with the hyphae, formed the exciple or thalline margin. The growth 
upward of the paraphyses raises the overlying cortex which in Anaptychia 
is " fibrous "; it gradually dies off and allows the exposure of the disc, though 
small shreds of dead tissue are frequently left. In species such as those of 
Xanthoria where the cortex is of vertical cell-rows, the apothecial hyphae 
simply push their way between the cell-rows and so through to the open. 

Baur found the development of the apothecium somewhat similar in the 
crustaceous corticolous lichen, Lecanora subfusca. After a long period of 
sterile growth, spermogonia appeared in great abundance, and, a little later, 
carpogonia in groups of five to ten ; the trichogynes emerged very slightly 
above the cortex; they were now branched. The ascogonia were frequently 
a confused clump of cells, though sometimes they showed distinct spirals. 
The surrounding hyphae had taken a vertical direction towards the cortex 
at a still earlier stage, and the brown tips were visible on the exterior before 
the trichogynes were formed. The whole growth was extremely slow. 

In Physcia stellaris the carpogonia occurred in groups also, though Lin- 
dau 1 thinks that, unlike Anaptychia (Physcia} ciliaris, only one is left to form 
the fruit. Only one, according to Darbishire 2 , entered into the apothecium 
in the allied species, Physcia pulverulenta. In the latter plasma connections 
were visible from cell to cell of the trichogyne, and, after copulation with 
the spermatium, the ascogonial cells increased in size though not in number 
and the plasma connections between them became so wide that the asco- 
gonium had the appearance of an almost continuous multinucleate cell or 
coenogamete 3 . As in gelatinous lichens, each of these cells gave rise to 
ascogenous hyphae. 

c. GENERAL SUMMARY. The main features of development described 
above recur in most of the species that have been examined. 

(i) The carpogonia arise in a complex of hyphae situated on the under 
side of, or immediately below the gonidial zone. Usually they vary in number 
from five to twenty for each apothecium, though as many as seventy-two 
have been computed for Icmadophila ericetorum*, and Wainio 5 describes 
them as so numerous in Coccocarpia pellita var., that their trichogynes covered 
some of the young apothecia with a hairy pile perceptible with a hand lens, 
though at the same time other apothecia on the same specimens were 
bsolutely smooth. 

1 Lindau 1888. 2 Darbishire 1900. 3 See also p. 180. 4 Nienburg 1908. 5 Wainio i. 1890. 




REPRODUCTION IN DISCOLICHENS 167 

(2) The trichogynes, when present, travel up through the gonidial and 
cortical regions of the thallus; Darbishire 1 observes that in Physcia pulveru- 
lenta, they may diverge to the side to secure an easier course between the 
groups of algae. They emerge above the surface to a distance of about 1 5/i 
or less; after an interval they collapse and disappear. Their cells, which are 
longer and narrower than those of the ascogonium, are uninucleate and vary 
in number according to species or to individual lichens. Baur 2 thought that 
possibly several trichogynes in succession might arise from one ascogonium. 

(3) How many carpogonia share in the development of the apothecium 
is still a debated question. In Collema only one is 

functional. Baur 3 was unable to decide if one or 
more were fertilized in Parmelia acetabulum, and 
in Usnea Nienburg 4 found that, out of several, one 
alone survived (Fig. 95). But in Anaptychia ciliaris 
and in Lecanora subfusca Baur 3 considers it proved 
that several share in the formation of the apothecium. 
In this connection it is interesting to note that, 
according to Harper 5 and others, several ascogonia 
enter into one Pyronema fruit. 

(4) The ascogonial cells, before and after ferti- 
lization, are distinguished from the surrounding F 'g; 95- Vsneabarbata\jl&>. 

~ Carpogonium with tricho- 

hyphae by a reaction to various stains, which is dif- gynex uoo (after Nien- 
ferent from that of the vegetative hyphae, and also bur s)- 
by the shortness and width of their cells. The whole of the apothecial primor- 
dium is generally recognizable by the clear shining appearance of the cells. 

(5) *The ascogonia do not always form a distinct spiral; frequently they 
lie in irregular groups. Each cell is uninucleate and may ultimately produce 
ascogenous hyphae, though in Anaptychia Baur 3 noted that some of the 
cells failed to develop. 

(6) The hyphae from the ascogonial cells spread out in a complex layer 
at the base of the hymenium, and send up branches which form the asci, 
either, as in most Ascomycetes, from the penultimate cell of the fertile branch, 
or from the last cell, as in Sphyridium (Baeomyces rufus)* and in Baeomyces 
roseus. The same variation has been observed in fungi in a species of 
Peziza 6 , in which it is the end-cell of the branch that becomes the mother- 
cell of the ascus; but this deviation from the normal is evidently of rare 
occurrence either in lichens or fungi. 

d. HYPOTHECIUM AND PARAPHYSES. The hypothecium is the layer of 
hyphae that subtends the hymenium, and is formed from the complex of 

1 Darbishire 1900. 2 Baur 1901. 3 Baur 1904. 4 Nienburg 1908. 

5 Harper 1900. 6 Guilliermond 1904, p. .60. 



1 68 REPRODUCTION 

hyphae that envelope the first stages of the carpogonia. It is vegetative in 
origin and distinct from the generative system. 

In lichens belonging to the Collemaceae, the paraphyses rise from the 
branching of the carpogonial stalk-cell immediately below the ascogonium 1 , 
but have no plasma connection with it. They are thus comparable in origin 
with the paraphyses of many Discomycetes. 

In several genera in which the algal constituents are blue-green, such as 
Stictina, Pannaria, Nephroma, Ricasolia and Peltigera, Sturgis 2 found that 
reproduction was apogamous and also that asci and paraphyses originated 
from the same cell-system : a tuft of paraphyses arose from the basal cell 
of the ascus, or an ascus from the basal cell of a paraphysis. These results 
are at variance with those of most other workers, but the figures drawn by 
Sturgis seem to be clear and convincing. 

Again in Usnea barbata, as described by Nienburg 3 ., the ascogonial cells, 
after the disappearance of the trichogyne, branch profusely not only up- 
wards towards the cortex but also downwards and to each side The upward 
branches give rise normally to the asci, the lower branches produce the sub- 
hymenium and later the paraphyses, and the two systems are thus genetically 
connected, though they remain distinct from each other, and asci are never 
formed from the lower cells. 

In most heteromerous lichens, however, the origin of the paraphyses 'is 
exclusively vegetative: they arise as branches from the primordial complex 
that forms the covering hyphae of the ascogonium both above and below. 
Schwendener 4 had already pointed out the difference in origin between the 
two constituents of the hymenium in one of his earlier studies on the de- 
velopment of the apothecium, and this view has been repeatedly confirmed 
by recent workers, except by Wahlberg 5 who has insisted that they rise from 
the same cells as the asci, a statement disproved by Baur 6 . The paraphyses 
originate not only from the covering hyphae, but from vegetative cells in 
close connection with the primordium. In this mode of development, lichens 
diverge from fungi, but even in these a vegetative origin for the paraphyses 
has been pointed out in Lachnea scutellata? where they branch from the 
hyphae lying round the ascogonium. 

There is no general rule for the order of development. In Lecanora sub- 
fusca Baur 6 found that vertical filaments had reached the surface by the time 
the trichogyne was formed, and their pointed brown tips gave a ready clue 
to the position of the carpogonia. In Lecidea enteroleuca* they show their 
characteristic form and arrangement before there is any trace of ascus 
formation. In Solorina* they are well formed before the ascogenous 
hyphae appear. In other lichens such as Placodium saxicolum*, Peltigera 

1 Baur 1899. Sturgis 1890. 3 Nienburg 1908. 4 Schwendener 1864. 5 Wahlberg 1902. 
6 Baur 1904. 7 Brown 1911. 8 Moreau 1916. 9 Lindau 1888. 



REPRODUCTION IN DISCOLICHENS 169 

rufescens 1 and P. malacea* the two systems paraphyses and ascogonium 
grow simultaneously, though in P. horizontalis the ascogonium has dis- 
appeared by the time the paraphyses are formed. In the genus Nephroma, 
in Physcia stellaris and in Xanthorina parietina the paraphyses are also late 
in making their appearance. 

In most instances, the paraphyses push their way up between the cortical 
cells which gradually become absorbed, or they may stop short of the sur- 
face as in Nephromium tomentosum*. The overlying layer of cortical cells in 
that case dies off gradually and in time disappears. Such an apothecium is 
said to be " at first veiled." Later formed paraphyses at the circumference 
of the apothecium form the parathecium, which is thus continuous with the 
hypothecium. 

e. VARIATIONS IN APOTHECIAL DEVELOPMENT. Lichens are among 
the least stereotyped of plants : instances of variation have been noted in 
several genera. 

aa. PARMELIAE. A somewhat complicated course of development has 
been traced by Baur 2 in Parmelia acetabulum. In that lichen the group of 
three to six carpogonia do not lie free in 
the gonidial tissue, but originate nearer 
the surface (Fig. 96) and are surrounded 
from the first by a tissue connected with, 
and resembling the tissue of the cortex. 
In the several ascogonia, there are more 
cells and more spirals than in Collema 
or in Physcia, and all of them are some- 
what confusedly intertwined. The tri- 
chogynes are composed of three to five 
cells and project 10 to I5ytt above the 
surface. When further development be- 
gins, the ascogonial cells branch out and 
form a primary darker layer or hypo- x 55 (after Baur). 
thecium above which extends the subhymenium, a light-coloured band of 
loosely woven hyphae. Branches from the ascogonial hyphae at a later stage 
push their way up through this tissue and form above it a second plexus of 
hyphae the base of the hymenium. Baur considers this a very advanced 
type of apothecium; he found it also present in Parmelia saxatilis, though, 
in that species, the further growth of the first ascogonial layer was more 
rapid and the secondary plexus and hymenium were formed earlier in the 
life of the apothecium. He has also stated that a similar development occurs 
in other genera such as Usnea, though Nienburg's 3 work scarcely confirms 
that view. 

1 Funfsttick 1884. * Baur 1904. 3 Nienburg 1908. 




i ;o 



REPRODUCTION 



In the brown Parmeliae, Rosendahl 1 found the same series of apothecial 
tissues, but he interprets the course of development somewhat differently: 
the basal dark layer or hypothecium he found to be of purely vegetative 
origin ; above it extended the lighter-coloured subhymenium ; the ascogenous 
hyphae were present only in the second layer of dark tissue immediately 
under the hymenium. 

In most lichens the primordium of the apothecium arises towards the 
lower side of the gonidial zone, the hyphae of which retain the meristematic 
character. In Parmeliae, as was noted by Lindau 2 in P. tiliacea, and by 
Baur 3 and Rosendahl 1 in other species, the carpogonial groups are formed 
above the gonidial zone, either immediately below the cortex as in P. glabra- 
tula, or in a swelling of the cortex itself as in P. aspidota, in which species 
the external enlargement is visible by the time the trichogynes reach the 
surface. In P. glabra, with a development entirely similar to that of P. as- 
pidota, no trichogynes were seen at any stage. The position of the primordium 
close under the cortex is also a feature of Ramalina fraxinea as described 
by G. Wolff 4 . The trichogynes in that species are fairly numerous. 

A further peculiarity in Parmelia acetabulum attracted Baur's 3 attention. 
Carpogonia with trichogynes are extremely numerous in that species as are 
the spermogonia, the open pores of which are to be found everywhere between 
the trichogynes, and yet fertilization can occur but rarely, as disintegrating 
carpogonia are abundant and very few apothecia are formed. Baur makes 
the suggestion that possibly cross-fertilization may be necessary, or that the 
spermatia, in this instance, do not fertilize and that development must 
therefore be apogamous, in which case the small number of fruits formed is 
due to some unknown cause. Fiinfstuck 5 thought that degeneration of the 
carpogonia had not gone so far, but that a few had acquired the power to 
develop apogamously. In Parmelia saxatilis only a small percentage of 
carpogonia attain to apothecia, although spermogonia are abundant and in 
close proximity, but in that species, unlike P. acetabulum, a large number 
reach the earlier stages of fruit formation ; the more vigorous apothecia seem 
to inhibit the growth of those that lag behind. 

bb. PERTUSARIAE. In Pertusaria, the apothecial primordium is situated 
immediately below the gonidial zone; the cells have a somewhat larger 
lumen and thinner walls than those of the vegetative hyphae. In the asco- 
gonium there are more cells than in Parmelia acetabulum] the trichogynes 
are short-lived, and several carpogonia probably enter into the formation of 
each apothecium ; the paraphyses arise from the covering hyphae. So far the 
course of development presents nothing unusual. The peculiar pertusarian 
feature as described by Krabbe 6 , and after him by Baur 7 , does not appear 

1 Rosendahl 1907. 2 Lindau 1888. 3 Baur 1904. 4 Wolff 1905. 

5 Funfstiick 1902. 6 Krabbe 1882. 7 Baur 1901. 



REPRODUCTION IN DISCOLICHENS 



171 



till a later stage. By continual growth in thickness of the overlying 
thallus, the apothecia gradually become submerged and tend to degenerate; 
meanwhile, however, a branch from the ascogonial hyphae at the base of 
the hymenium pushes up along one side and forms a secondary ascogonial 
cell-plexus over the top of the first-formed disc. A new apothecium thus 
arises and remains sporiferous until it also comes to lie in too deep a position, 
when the process is repeated. Sometimes the regenerating hypha travels to 
the right or left away from the original apothecium, it may be to a distance 
of 2 mm. or according to Fiinfstiick even considerably farther. Funfstiick 1 has 
gathered indeed from his own investigations that such cases of regeneration 
are by no means rare : ascogenous hyphae, several centimetres long, destined 
to give rise to new apothecia are not unusual, and their activity can be recog- 




Fig. 97. Rhizocarpon petrae um Massal. Concentrically arranged apothecia, reduced 
(J. Adams, Photo.}. 

nized macroscopically by the linear arrangement of the apothecia in such 
lichens as RJiizocarpon (petraeuwi) concentricum (Fig. 97). 

In Variolaria, a genus closely allied to or generally included in Per- 
tusaria, Darbishire 2 has described the primordial tissue as taking rise almost 
at the base of the crustaceous thallus: strands of delicate hyphae, staining 

1 Ftinfstiick 1902. 2 Darbishire 1897. 



172 



REPRODUCTION 



blue with iodine, mount upwards from that region through the medulla and 
gonidial zone 1 . The ascogonium does not appear till the surface is almost 
reached. 

cc. GRAPHIDEAE. Several members of the Graphidaceae were studied 
by G. Wolff 2 : she demonstrated the presence of carpogonia with emerging 
trichogynes in Graphis elegans, a species which is distinguished by the deeply 
furrowed margins of the lirellae (Fig. 89). Before the carpogonia appeared 
it was possible to distinguish the cushion-like primordial tissue of the apo- 
thecium in the thallus which is almost wholly immersed in the periderm 
layers of the bark on which it grows. The trichogynes were very sparingly 
septate, and a rather large nucleus occupied a position near the tip of the 
terminal cell. The dark carbonaceous outer wall makes its appearance in 
this species at an early stage of development along the sides of the lirellae, 
but never below, as there is always a layer of living cells at the base. After 
the first-formed hymenium is exhausted, these basal cells develop a new 
apothecium with a new carbonaceous wall that pushes back the first-formed, 
leaving a cleft between the old and the new. This regenerating process, 
somewhat analogous to the formation of new apothecia in Pertusaria, may 
be repeated in Graphis elegans as many as five times, the traces of the older 
discs being clearly seen in the channelled margins of the lirellae. 

dd. CLADONIAE. The chief points of interest in the Cladoniae are the 
position of the apothecial primordia and the function of the podetium, 

which are discussed later 3 . Krabbe 4 deter- 
mined not only the endogenous origin of 
the podetium but also the appearance of 
fertile cells in the primordium (Fig. 98). 
Both frequently take rise where a crack 
occurs in the cortex of the primary squa- 
mule, the cells of the gonidial tissue being 
especially active at these somewhat ex- 
posed places. The fertile hyphae elongate 
and branch within the stalk of the de- 
veloping podetium, sometimes very early, 
or not until there is a pause in growth, 
when carpogonia are formed. As a rule 
trichogynes emerge in great numbers 2 , generally close to, or rather below, 
the spermogonia. In Cl. pyxidata* the carpogonia are characterized by the 
large diameter of the cells three to five times that of the vegetative hyphae. 
Though most of the trichogynes disappear at an early stage, some of them 
may persist for a considerable period. As development proceeds, the vege- 
tative hyphae interspersed among the ascogonial cells grow upwards, slender 

1 See also p. 147. Wolff 1905. 3 See Chap. VII. Krabbe 1883 and 1891. s Baur 1904. 




d 



Fig. 98. Cladonia decorticata Spreng. Ver- 
tical section of squamule and primordium 
of podetium. a, developing podetium; 
b, probably fertile hyphae; c, cortical 
tissue ; </, gonidial cells, j x 600 (after 
Krabbe). 



REPRODUCTION IN DISCOLICHENS 



173 



branches push up between them and gradually a compact sheath of para- 
physes is built up. The ascogenous hyphae meanwhile spread radially at 
the base of the paraphyses and the asci begin to form. The apothecia may 
be further enlarged by intercalary growth, and this vigorous development 
of vegetative tissue immediately underneath raises the whole fruit structure 
well above the surface level. 

Sattler 1 in his paper on Cladoniae* cites as an argument in favour of 
fertilization the relative positions of carpogonia and spermogonia on the 
podetia. The carpogonia with their emerging trichogynes being situated 
rather below the spermogonia. Both organs, he states, have been demon- 
strated in eleven species; he himself observed them in the primordial podetia 
of Cladonia botrytes and of Cl. Floerkeana. 

2. PYRENOLICHENS 

a. DEVELOPMENT OF THE PERITHECIUM. It is to Fuisting 3 that we 
owe the first account of development in the lichen perithecium. Though 
he failed to see the earlier stages (in Verrucaria Dufourii), he recognized 
the primordial complex of hyphae in the gonidial zone of the thallus, from 
which originated a vertical strand of hyphae destined to form the tubular 
neck of the perithecium. Growth in the lower part is in abeyance for 
a time, and it is only after the neck is formed, and the fruiting body is 
widened by the ingrowth of external hyphae, that the asci begin to branch 
up from the tissue at the base. 

b. FORMATION OF CARPOGONIA. Stahl 4 had indicated that not only 
in gymnocarpous but also in angiocarpous 

lichens, it would be found that carpo- 
gonia were formed as in Collema. Baur 3 
justified this surmise, and demonstrated the 
presence of ascogonia in groups of three to 
eight, with trichogynes that reached the 
surface in Endocarpon {Dermatocarpon) mi- 
niatum (Fig. 99). It is one of the few 
foliaceous Pyrenolichens, and the leathery 
thallus is attached to the substratum by a 
central point, thus allowing in the thallus 
not only peripheral but also intercalary 
growth, the latter specially active round the 
point of basal attachment; carpogonia may 

be found in any region where the tissue is Fig. 99. Dermatocarpon miniatum 
, 3 . . Th. Fr. Vertical section of thallus 

newly formed, and at any season. I he upper 




cortex is composed of short-celled thick- 
1 Sattler 1914. ~ See Chap. VII. 3 Fuisting 18 



and carpogonial group x 600 (after 
Baur). 



4 Stahl 1877. 



Baur 1904. 



i 7 4 REPRODUCTION 

walled hyphae, with branching vertical to the surface, and so closely packed 
that there is an appearance of plectenchyma ; the medullary hyphae are 
also thick-walled but with longer cells. The carpogonia of this species 
arise as a branch from the vegetative hyphae and are without special covering 
hyphae, so frequent a feature in other lichens. The trichogynes bore their 
nay through the compact cortex and rise well above the surface. After they 
have disappeared presumably after fertilization the vegetative hyphae 
round and between the ascogonia become active and travel upwards slightly 
converging to a central point. The asci begin to grow out from the asco- 
genous hyphae of the base before the vertical filaments have quite pierced 
the cortex. 

Pyrenula nitida has also been studied by Baur 1 . It is a very common 
species on smooth bark, with a thin crustaceous thallus immersed among 
the outer periderm cells. Unlike most other lichens, it forms carpogonia 
in spring only, from February to April. A primordial coil of hyphae lies at 
the base of the gonidial layer, and, before there is any appearance of carpo- 
gonia, a thick strand of hyphae is seen to be directed upwards, so that a 
definite form and direction is given to the perithecium at a very early stage. 
The ascogonial cells which are differentiated are extremely small, and, like 
those of all other species examined, are uninucleate. There are five to ten 
carpogonia in each primordium ; the trichogynes grow up through the hyphal 
strand and emerge 5-10 /* above the surface. After their disappearance, a 
weft of ascogenous tissue is formed at the base, and, at the same time, the 
surrounding vegetative tissue takes part in the building up of a plecten- 
chymatous wall of minute dark-coloured cells. Further development is 
rapid and occupies probably only a few weeks. 

In many of the pyrenocarpous lichens Verrucariae and others the 
walls of the paraphyses dissolve in mucilage as the spores become mature, 
a character associated with spore ejection and dispersal. In some genera 
and species, as in Pyrenula, they remain intact. 

D. APOGAMOUS REPRODUCTION 

Though fertilization by an externally produced male nucleus has not 
been definitely proved there is probability that, in some instances, the fruit 
may be the product of sexual fusion. There are however a number of genera 
and species in which the development is apogamous so far as any external 
copulation is possible and the sporiferous tissue seems to be a purely vege- 
tative product up to the stage of ascus formation. 

In Phlyctis agelaea Krabbe 2 found abundant apothecia developing nor- 
mally and not accompanied by spermogonia; in Phialopsis rubra studied 

1 Baur 1901. 2 Krabbe 1882. 



APOGAMOUS REPRODUCTION 175 

also by him the primordium arises among the cells of the periderm on which 
the lichen grows, and he failed to find any trace of a sexual act. In his 
elaborate study of Gloeolichens Forssell 1 established the presence of carpo- 
gonia with trichogynes in two species Pyrenopsis phaeococca and P. impolita, 
but without any appearance of fertilization; in all the others examined, the 
origin of the fruit was vegetative. Wainio 2 records a similar observation in 
a species of Pyrenopsis in which there was formed a spiral ascogonium and 
a triehogyne, but the latter never reached the surface. 

Neubner 3 claimed to have proved a vegetative origin for the asci in the 
Caliciaceae; but he overlooked the presence of spermogonia and his conclu- 
sions are doubtful. 

Fiinfstuck 4 found apogamousdevelopment inPeltigera(\r\c\\idmgPeltidea) 
and his results have never been disputed. The ascogonial cells are surrounded 
at an early stage by a weft of vegetative hyphae. No trichogynes are formed 
and spermogonia are absent or very rare in the genus, though pycnidia with 
macrospores occur occasionally. 

Some recent work by Darbishire 5 on the genus supplies additional details. 
The apothecial primordium always originated near the growing margin of 
the thallus, where certain medullary hyphae were seen to swell up and stain 
more deeply than others. These at first were uninucleate, but the nuclei 
increased by division as the cells became larger, and in time there was 
formed a mass of closely interwoven cells full of cytoplasm. " No coiled 
carpogonia can be made out, but these darkly stained cells form part of a 
connected system of branching hyphae coming from the medulla further 
back." Long unbranched multiseptate hyphae evidently functionless tri- 
chogynes travelled towards the cortex but gradually died off. Certain of 
the larger cells the " ascogonia " grew out as ascogenous hyphae into 
which the nuclei passed in pairs and finally gave rise to the asci. 

These results tally well with those obtained by M. and Mme Moreau 6 , 
though they make no mention of any triehogyne. They found that the 
terminal cells of the ascogenous hyphae were transformed into asci, and the 
two nuclei in these cells fused the only fusion that took place. In Nephro- 
mtum, one of the same family, the case for apogamy is not so clear; but 
Fiinfstuck found no trichogynes, and though spermogonia were present on 
the thallus, they were always somewhat imperfectly developed. 

Sturgis 7 supplemented these results in his study of other lichens con- 
taining blue-green algae. In species of Heppia, Pannarta, Hydrothyria, 
Stictina and Ricasolia, he failed to find any evidence of fertilization by 
spermatia. 

Solorina, also a member of Peltigeraceae, was added to the list of 

1 Forssell 1885. 2 Wainio 1890, p. x. 3 Neubner 1893. 4 Fiinfstuck 1884. 

5 Darbishire 1913. 6 Moreau 1915. 7 Sturgis 1890. 



i 7 6 REPRODUCTION 

apogamous genera by Metzger 1 and his work was confirmed and amplified 
by Baur 2 : certain hyphae of the gonidial zone branch out into larger asco- 
gonial cells which increase by active intercalary growth, by division and by 
branching, and so gradually give rise to the ascogenous hyphae and finally 
to the asci. Baur looked on this and other similar formations as instances 
of degeneration from the normal carpogonial type of development. Moreau 3 
(Fernand and Mme) have also examined Solorina with much the same 
results: the paraphyses rise first from cells that have been produced by the 
gonidial hyphae; later, ascogenous hyphae are formed and spread horizontally 
at the base of the paraphyses, finally giving rise at their tips to the asci. 
Metzger 1 had further discovered that spermogonia were absent and tricho- 
gynes undeveloped in two very different crustaceous lichens, Acarospora 
(Lecanora) glaucocarpa and Verrucaria calciseda, the latter a pyrenocarpous 
species and, as the name implies, found only on limestone. 

Krabbe 4 had noted the absence of any fertilization process in Gyrophora 
vellea. At a later date, Gyrophora cylindrica was made the subject of exact 
research by Lindau 5 . In that species the spermogonia (or pycnidia) are 
situated on the outer edge of the thallus lobes; a few millimetres nearer the 
centre appear the primordia of the apothecia, at first without any external 
indication of their presence. The initial coil which arises on the lower side 
of the gonidial zone consists of thickly wefted hyphae with short cells, slightly 
thicker than those of the thallus. It was difficult to establish their connec- 
tion with the underlying medullary hyphae since these very soon change to a 
brown plectenchyma. From about the middle of the ascogonial coil there 
rises a bundle of parallel stoutish hyphae which traverse the gonidial zone 
and the cortex and slightly overtop the surface. They are genetically con- 
nected at the base with the more or less spirally coiled hyphae, and are similar 
to the trichogynes described in other lichens. Lindau did not find that they 
had any sexual significance, and ascribed to them the mechanical function of 
terebrators or borers. The correctness of his deductions has been disputed by 
various workers: Baur 2 looks on these "trichogynes" as the first paraphyses. 
The reproductive organs in Stereocaulon were examined by G. Wolff 6 , and 
the absence of trichogynes was proved, though spermogonia were not wanting. 
She also failed to find any evidence of fertilization in Xanthoria parietina, 
in which lichen the ascogenous hyphae branch out from an ascogonium that 
does not form a trichogyne. 

Rosendahl 7 , as already stated, could find no trichogynes in Parmelia 
glabra. In Parmelia obscurata, on the contrary, Bitter 8 found that carpogonia 
with trichogynes were abundant and spermogonia very rare. In other species 
of the subgenus, Hypogymnia, he has pointed out that apothecia are either 

1 Metzger 1903. 2 Baur 1904. 3 Moreau 1916. 4 Krabbe 1882. 5 Lindau 1899. 
6 Wolff 1905. 7 Rosendahl 1907. 8 Bitter i9oi 2 . 



DISCUSSION OF LICHEN REPRODUCTION 177 

absent or occur but seldom, while spermogonia are numerous, and he concludes 
that the spermatia must function as spores or conidia. Baur 1 however does 
not accept that conclusion; he suggests as probable that the male organs 
persist longer in a functionless condition than do the apothecia. 

Still more recently Nienburg 2 has described the ascogonium of Baeo- 
myces sp. and also of Sphyridium byssoides (Baeomyces rufus) as reduced 
and probably degenerate. His results do not disprove those obtained by 
Krabbe 3 on the same lichen {Sphyridium fungiforme). The apothecia are 
terminal on short stalks in that species. When the stalk is about '5 mm. in 
height, sections through the tip show numerous primordia (12 to 15) ranged 
below the outer cortex, though only one, or at most three, develop further. 
These ascogonial groups are connected with each other by delicate filaments, 
and Nienburg concluded that they were secondary products from a primary 
group lower down in the tissue. Spirals were occasionally seen in what he 
considered to be the secondary ascogonia, but usually the fertile cells lie in 
loose uncoiled masses; isolated hyphae were observed to travel upwards 
from these cells, but they never emerged above the surface. 

Usnea macrocarpa if Schulte's 4 work may be accepted is also apo- 
gamous, though in Usnea barbata Nienburg 2 found trichogynes (Fig. 95) 
and the various developments that are taken as evidence of fertilization. 
Wainio 5 had demonstrated emergent straight trichogynes in Usnea laevis 
but without any sign of fertilization. 

E. DISCUSSION OF LICHEN REPRODUCTION 

In Ascolichens fertilization by the fusion of nuclei in the ascogonium 
is still a debated question. The female organ or carpogonium, as outlined 
above, comprises a twisted or spirally coiled multiseptate hypha, with a 
terminal branch regarded as a trichogyne which is also multiseptate, and 
through which the nucleus of the spermatium must travel to reach the 
female cell. It is instructive to compare the lichen carpogonium with that of 
other plants. 

a. THE TRICHOGYNE. In the Florideae, or red seaweeds, in which the 
trichogyne was first described, that organ is merely a hair-like prolongation 
of the egg-cell and acts as a receptive tube. It contains granular proto- 
plasm but no nucleus and terminates in a shiny tip covered with mucilage. 
The spermatium, unlike that of lichens, is a naked cell, and being non-motile 
is conveyed by water to the tip of the trichogyne to which it adheres; the 
intervening wall then breaks down and the male nucleus passes over. After 
this process of fertilization a plug of mucilage cuts off the trichogyne, and 
it withers away. 

1 Baur 1904. - Xienburg 1908. 3 Krabbe 1882. 4 Schulte 1904. 5 Wainio 1890. 
S. L. 12 



i 7 8 REPRODUCTION 

In Coleochaete, a genus of small fresh- water green algae, a trichogyne is 
also present in some of the species: it is again a prolongation of an oogonial 
cell. 

In the Ascomycetes, certain cells or cell-processes associated with the 
ascogonium have been described as trichogynes or receptive cells. In one 
of the simpler types, Monascus 1 , the " trichogyne" is a cell cut off from the 
ascogonial cell. When fertilization takes place, the wall between the two 
cells breaks down to allow the passage of the male nucleus, but closes up 
when the process is effected. In Pyronema confluens* it is represented by a 
process from the ascogonial cell which fuses directly with the male cell. A 
more elaborate "trichogyne " has been evolved in Lachnea stercorea*, another 
Discomycete: in that fungus it takes the form of a 3~5-septate hypha with 
a longer terminal cell; it rises from some part of the ascogonial cell but has 
no connection with any process of fertilization, so that the greater elaboration 
of form is in this case concomitant with loss of function. 

In the Laboulbeniaceae, a numerous and very peculiar series of Asco- 
mycetes that live on insects, there are, in nearly all of the reproductive bodies, 
a carpogonial cell, a trichophoric cell and a trichogyne. The last-named 
organ is in some genera a simple continuous cell, in others it is septate and 
branched, occasionally it is absent 4 . The male cells are spermatia of two 
kinds, exogenous or endogenous, and the plants are monoecious or dioecious. 
Laboulbeniaceae have no connection with lichens. Faull 5 , a recent worker 
on the group, states that though he observed spermatia attached to the tri- 
chogynes, he was not able to demonstrate copulation (possibly owing to 
over-staining), nor could he trace any migration of the nucleus through the 
trichophoric cell down to the carpogonial cell. In two species of Labotdbenia 
that he examined there were no antheridia, and the egg-cell acquired its 
second nucleus from the neighbouring trichophoric cell. These conjugate 
nuclei divided simultaneously and the two daughter nuclei passed on to the 
ascus and fused, as in other Ascomycetes, to form the definitive nucleus. 

Convincing evidence as to the importance of the trichogyne in fungi was 
supposed, until lately, to be afforded by the presence and functional activity 
of that organ associated with spermogonia in a few Pyrenomycetes in 
Poronia, Gnomonia and Polystigma. Poronia was examined by M. Dawson 6 
who found that a trichogyne-like filament distinct from the vegetative hyphae 
rose from the neighbourhood of the ascogonial cells. It took an upward 
course towards the exterior, but there was no indication that it was ever 
receptive. In Gnomonia erythrostoma and in Polystigma rubrum spermogonia 
with spermatia presumably male organs are produced in abundanceshortly 
before the ascosporous fruit is developed. The spermatia in both cases exhibit 

1 Schikorra 1909. 2 Harper 1900. 3 Fraser 1907. 4 Thaxter 1912. 

5 Faull 1911. 6 Dawson 1900. 



DISCUSSION OF LICHEN REPRODUCTION 179 

the characters of male cells, i.e. very little cytoplasm and a comparatively large 
nucleus that occupies most of the cell cavity, along with complete incapacity 
to germinate. Brooks 1 found in Gnomonia that tufts of the so-called tricho- 
gynes originated near the ascogonial cells, but they were " mere continuations 
of ordinary vegetative hyphae belonging to the coil." They are septate and 
reach the surface, and the tip-cell is longer than the others as in the lichen 
trichogyne. 

A somewhat similar arrangement is present in Polystigma, in which 
Blackman and Welsford 2 have proved that the filaments, considered as tri- 
chogynes by previous workers, are merely vegetative hyphae. A trichogyne- 
like structure is also present in Capnodium, one of the more primitive Pyreno- 
mycetes, but it has no sexual significance. 

Lindau 3 in his paper on Gyrophora suggested that the trichogyne in 
lichens acted as a " terebrator " or boring apparatus, of service to the deeply 
immersed carpogonium in enabling it to reach the surface. Van Tieghem 4 
explained its presence on physiological grounds as necessary for respiration, 
a view also favoured by Zukal 5 , while Wainio 6 and Steiner 7 see in it only an 
" end-hypha," the vigorous growth of which is due to its connection with 
the well-nourished cells of the ascogonium. 

Lindau's view has been rejected by succeeding writers: as has been 
already stated, it is the paraphyses that usually open the way outward for 
the apothecium. Van Tieghem's theory has been considered more worthy 
of attention and both Dawson and Brooks incline to think that the projecting 
filaments described above may perform some service in respiration, even 
though primarily they may have functioned as sexual receptive organs. 

There is very little support to be drawn from fungi for the theory that 
the presence of a trichogyne necessarily entails fertilization by spermatia. 
Lichens in this connection must be judged as a class apart. 

It has perhaps been too lightly assumed that the trichogyne in lichens 
indicates some relationship with the Florideae 8 . Such a view might be possible 
if we could regard lichens and Florideae as derived from some common 
remote ancestor, though even then the difference in spore production in 
one case exogenous, and in the other in asci and therefore endogenous 
would be a strong argument against their affinity. But all the evidence goes 
to prove that lichens are late derivatives of fungi and have originated from 
them at different points. Fungi are interposed between lichens and any 
other ancestors, and inherited characters must have been transmitted through 
them. F. Bachmann's suggestion 9 that Collema pulposum should be regarded 
" as a link between aquatic red algae and terrestrial ascomycetes such as 
Pyronema and the mildews " cannot therefore be accepted. It seems more 

1 Brooks 1910. 2 Blackman and Welsford 1912. 3 Lindau 1899. * Van Tieghem 1891. 
5 Zukal 1895. 6 Wainio 1890. 7 Steiner 1901. 8 See also Chap. VII. 9 F. Bachmann 1913. 



i8o REPRODUCTION 

probable that the lichen trichogyne is a new structure evolved in response 
to some physiological requirement either sexual or metabolic of the deeply 
embedded fruit primordium. 

b. THE ASCOGONIUM. In fungi there is usually one cell forming the 
ascogonium, a coenogamete, which after fertilization produces ascogenous 
hyphae. There are exceptions, such as Cutting 1 found in Ascophanus carneus, 
in which it is composed of several cells in open contact by the formation of 
wide secondary pores in the cell-walls. In lichens the ascogonium is divided 
into a varying number of uninucleate cells. Darbishire 2 (in Physcia) and 
Baur 3 (in Anaptychia) have described an opening between the different cells, 
after presumed fertilization, that might perhaps constitute a coenogamete. 
Ascogenous hyphae arise from all, or nearly all the cells, whether fertilized by 
spermatia or not, and asci continue to be formed over a long period of time. 
There may even be regeneration of the entire fruiting body as described in 
Graphis elegans and in Pertusaria, apparently without renewed fertilization. 

Spermogonia (or pycnidia) and the ascosporous fruits generally grow on 
the same thallus, though not unfrequently only one of the two kinds is 
present. As the spermogonia appear first, while the apothecia or perithecia 
are still in the initial stages, that sequence of development seems to add 
support to the view that their function is primarily sexual; but it is equally 
valid as a proof of their pycnidial nature since the corresponding bodies in 
fungi precede the more perfect ascosporous fruits in the life-cycle. 

The differences in fertility between the two kinds of thallus in Collema 
crispum may be recalled 4 . Baur considered that development of the carpo- 
gonia was dependant on the presence of spermatia: a strong argument for 
the necessity of fertilization by these. The conditions in Parmelia acetabulum, 
also recorded by Baur, lend themselves less easily to any conclusion. On 
the thallus of that species the spermogonia and carpogonia present are out 
of all proportion to the very few apothecia that are ultimately formed. 
Though Baur suggested that cross-fertilization might be necessary, he admits 
that the development may be vegetative and so uninfluenced by the presence 
or absence of spermatia. 

It is the very frequent occurrence of the trichogyne as an integral part of 
the carpogonium that constitutes the strongest argument for fertilization by 
spermatia. There is a possibility that such an organ may have been uni- 
versal at one time both in fungi and in lichens, and that it has mostly 
degenerated through loss of function in the former, as it has disappeared in 
many instances in lichens. Again, there is but a scanty and vestigial record 
of spermogonia in Ascomycetes. They may have died out, or they may 
have developed into the asexual pycnidia which are associated with so many 
species. If we take that view we may trace the same tendency in lichens, as 

1 Cutting 1909. 2 Darbishire 1900. 3 Baur I9<>4> 4 See p . l6l . 



DISCUSSION OF LICHEN REPRODUCTION 181 

for instance in the capacity of various spermatia to germinate, though in 
lichen spermogonia there has been apparently less change from the more 
primitive condition. It is also possible that some process of nuclear fusion, 
or more probably of conjugation, takes place in the ascogonial cells, and 
that in the latter case the only fusion, as in some (or most) fungi, is between 
the two nuclei in the ascus. 

If it be conceded that fully developed carpogonia with emergent tricho- 
gynes, accompanied by spermogonia and spermatia, represent fertilization, 
or the probability of fertilization, then the process may be assumed to take 
place in a fairly large and widely distributed series of lichens. Copulation 
between the spermatium and the trichogyne has been seen by Stahl 1 , Baur 2 
and by F. Bachmann 3 in Collema. In Physcia pulverulenta Darbishire 4 could 
not prove copulation in the earlier stages, but he found what he took to be 
the remains of emptied spermatia adhering to the tips of old trichogynes. 
Changes in the trichogyne following on presumed copulation have been 
demonstrated by several workers in the Collemaceae, and open communi- 
cation as a result of fertilization between the cells of the ascogonium has 
been described in two species. This coenocytic condition of the ascogonium 
(or archicarp), considered by Darbishire and others as an evidence of fer- 
tilization, has been demonstrated by Fitzpatrick 8 in the fungus Rhizina 
undulata. The walls between the cells of the archicarp in that Ascomycete 
became more or less open, so that the ascogenous hyphae growing from the 
central cells were able easily to draw nutrition from the whole coenocyte, 
but no process of fertilization in Rhizina preceded the breaking down of the 
septa and no fusion of nuclei was observed until the stage of ascus. formation. 

The real distinction between fertile and vegetative hyphae lies, according 
to Harper 6 , in the relative size of the nuclei. F. Bachmann speaks of one 
large nucleus in the spermatium of Collema pulposum which would indicate 
sexual function. There is however very little nuclear history of lichens known 
at any stage until the beginning of ascus formation, when fusion of two nuclei 
certainly take place as in fungi to form the definitive nucleus of the ascus. 

The whole matter may be summed up in Fiinfstiick's 7 statement that: 
" though research has proved as very probable that fertilization takes place, 
it is an undoubted fact that no one has observed any such process." 

F. FINAL STAGES OF APOTHECIAL DEVELOPMENT 

The emergence of the lichen apothecium from the thallus, and the form 
it takes, are of special interest, as, though it is essentially fungal in structure, 
it is subject to various modifications entailed by symbiosis. 

1 Stahl 1877. 2 Baur 1898. 3 F. Bachmann 1912 and 1913. 4 Darbishire 1900. 
5 Fitzpatrick 1918. 6 Harper 1900. 7 Fiinfstttck 1902. 



Ig2 REPRODUCTION 

a. OPEN OR CLOSED APOTHECIA. Schwendener 1 drew attention to two 
types of apothecia directly influenced by the thallus: those that are closed 
at first and only open gradually, and those which are, as he says, open from 
the first. The former occur in genera and species in which the thallus has a 
stoutish cortex, as, for instance, in Lobaria where the young fructification 
has all the appearance of an opening perithecium. The open apothecia 
(primitus apertd) are found in non-corticate lichens, in which case the pioneer 
paraphyses arrive at the surface easily and without any converging growth. 
Similar apothecia are borne directly on the hypothallus at the periphery, or 
between the thalline areolae, and they are also characteristic of thin or slender 
thalli as in Coenogonium. 

In both types of apothecium, the paraphyses .pierce the cortex (Fig. 100) 
and secure the emergence of the developing ascomata. 




Fig. 100. Physcia ciliaris DC. Vertical section of apothe- 
cium still covered by the cortex, a, paraphyses ; b, hypo- 
thecium ; c, gonidia of thallus and amphithecium. x 150 
(after Baur). 

b. EMERGENCE OF THE ASCOCARP. Hue 2 has taken up this subject in 
recent years and has described the process by which the vegetative hyphae 
surrounding the fruit primordium, excited to active growth by contact with 
the generative system, take part in the later stages of fruit formation. The 
primordium generally occupies a position near to, or just within, the upper 
medulla, and the hyphae in contact with it soon begin to branch freely in a 
vertical direction, surrounding the developing fruit and carrying it upwards 
generally to a superficial position. The different methods of the final emer- 
gence give two very distinct types of mature apothecium: the lecideine in 
which the gonidial zone takes no part in the upward growth, and the leca- 
norine into which the gonidia enter as an integral part. 

In the lecideine series (Fig. 101) the encircling hyphae from the upper 
medulla rise as a compact column through the gonidial zone to the surface 
of the thallus ; they then spread radially before curving up to form the outer 
1 Schwendener 1864. 2 Hue 1906. 



DEVELOPMENT OF APOTHECIA 



183 




wall or " proper margin " round the spore-bearing disc. The branching of 
the hyphae is fastigiate with compact 
shorter branches at the exterior. In 
such an apothecium gonidia are ab- 
sent both below thehypothecium and 
in the margins. 

In lecanorine development the 
ascending hyphae from the medulla, 
in some cases, carry with them algal 
cells which multiply and spread as a second gonidial layer under the hypo- 
thecium (Fig. 102). These hyphae may also spread in a radial direction 
while still within the thallus and give rise to an " immersed " apothecium 
which is lecanorine as it encloses gonidia within its special tissues, for 
example, in Acarospora and Solorina. But in most cases the lecanorine fruit 
is superficial and not unfrequently it is raised on a short stalk (Usnea, etc.); 



Fig. 10 1. Lecidea parasema Ach. Vertical section 
of thallus and apothecium with proper margin 
only x ca. 50. 







Fig. 102. Lecanora far/area Ach. Vertical section of apo- 
thecium. a, hymenium ; b, proper margin or parathecium ; 
c, thalline margin or amphithecium. x 30 (after Reinke). 

both the primary gonidial zone of the thallus and the outer cortex are asso- 
ciated with the medullary column of hyphae from the first and grow up 
along with it, thus providing the outer part of the apothecium, an additional 
" thalline margin " continuous with the thallus itself. It is an advanced 
type of development peculiar to lichens, and it provides for fertility of long 
continuance which is in striking contrast with the fugitive ascocarps of the 
Discomycetes. 

The distinction between lecideine and lecanorine apothecia is of great 
value in classification, but it is not always easily demonstrable; it is 
occasionally necessary to examine the early stages, as in the more advanced 
the thalline margin may be pushed aside by the turgid disc and become 
practically obliterated. 




I84 REPRODUCTION 

The " proper margin " reaches its highest development in the lecideine 
and graphideine types. It is less prominent or often almost entirely replaced 
when the thalline margin is superadded, except in genera such as Thelotrema 
and Diploschistes which have distinct " double margins." 

There is an unusual type of apothecium in the genus Gyrophora. The 
fruit is lecideine, the thalline gonidia taking no part in the development. 

The growth of the initial ascogenous tissue, 
according to Lindau 1 , is constantly towards 
the periphery of the disc so that a weak 
spot arises in the centre which is promptly 
filled by a vigorous sterile growth of para- 
^^^^ , physes. This process is repeated from new 

Fig. 103. Apothecial gyrose discs of r J 

Gyrophora cylindrica. Ach. x 12 (after centres again and again, resulting in the 
Lmdau )- irregularly concentric lines of sterile and 

fertile areas of the "gyrose" fruit (Fig. 103). The paraphyses soon become 
black at the tips. Asci are not formed until the ascogenous layer has ac- 
quired a certain degree of stability, and spores are accordingly present only 
in advanced stages of growth. 

G. LICHEN ASCI AND SPORES 

a. HISTORICAL. The presence of spores, as such, in the lichen fruit was 
first established by Hedwig 2 in Anaptychia (Physcia) ciliaris. He rightly 
judged the minute bodies to be the "semina" of the plant. In that species 
they are fairly large, measuring about 50 /A long and 24 /j, thick, and as they 
are very dark in colour when mature, they stand out conspicuously from the 
surrounding colourless tissue of the hymenium. Acharius 3 also took note of 
these "semina" and happily replaced the term by that of "spores." They 
may be produced, he states, in a compact nucleus {Sphaerophoron\ in a naked 
disc (Calicium), or they may be embedded in the disc (Opegrapha and'Leadea). 
Sprengel 4 opined that the spores which he figures were true seeds, though 
he allows that there had been no record of their development into new plants. 
Luyken 5 made a further contribution to the subject by dividing lichens into 
gymnocarpous and angiocarpous forms, according as the spores, enclosed 
in cells or vesicles (thecae), were borne in an open disc or in a closed peri- 
thecium. 

In his Systema of lichen genera Eschweiler 6 , some years later, described 
and figured the spores as " thecae " enclosed in cylindrical asci. FeV in 
contemporary works gave special prominence to the colour and form of the 
spores in all the lichens dealt with. 

1 Lindau 1899. ' 2 Hedwig 1784. 3 Acharius 1803. 4 Sprengel 1807. 

5 Luyken 1809. 6 Eschweiler 1824. 7 Fee 1824. 



LICHEN ASCI AND SPORES 



185 




Hi^^lBr ' 

m 



b. DEVELOPMENT OF THE ASCUS. The first attempt to trace the origin 
and development of lichen asci and spores was made by Mohl 1 . He describes 
the mother-cell (the ascus) as filled at first with a clouded granular sub- 
stance changing later into a definite number usually eight of simple or sep- 
tate spores. Dangeard 2 included the lichens Borrera {Physcia} ciliaris and 
Endocarpon (Dermatocarpon) miniatum among the plants that he studied 
for ascus and spore development. He found that in lichens, as in fungi, the 
ascus arose usually from the penultimate cell of a crooked hypha (Fig. 104) 
and that it contained at first two nuclei 
derived from adjoining cells. These nuclei 
are similar in size to those of the vegetative 
hyphae, and in each there is a large nucleo- 
lus with chromatin material massed on one 
side. Fusion takes place, as in fungi, between 
the two nuclei, and the secondary or defi- 
nitive nucleus thus formed divides suc- 
cessively to form the eight spore-nuclei. 
Baur 3 and Nienburg 4 have confirmed Dan- 
geard's results as regards lichens, and Ren 
Maire 5 has also contributed important cyto- 
logical details on the development of the 
spores. In Anaptychia {Physcia) ciliaris he 
found that the fused nucleus became larger 
and that a synapsis stage supervened during 
which the long slender chromatin filaments 
became paired, and at the same time shorter and thicker. The nuclear mem- 
brane disappeared as the chromatin filaments were united in masses joined 
together by linin threads which also disappeared later. At the most advanced 
stage observed by Maire there was visible a nucleolus embedded in a con- 
densed plasma and surrounded by eight medianly constricted filaments 
destined to form the equatorial plate. A few isolated observations were also 
made on the cytology of the ascus in Peltigera canina, in which lichen the 
preceding ascogonial development is wholly vegetative. The secondary 
nucleus was seen to contain a chromatin mass and a large nucleolus; in 
addition two angular bodies of uncertain signification were associated with 
the nucleolus, each with a central vacuole. The nucleolus disappeared in the 
prophase of the first division and four double chromosomes were then plainly 
visible. The succeeding phases of the first and the second nuclear division 
were not seen, but in the prophase of the third it was possible to distinguish 
four chromatin masses outside the nucleolus. The slow growth of the lichen 
plant renders continuous observation extremely difficult. 

1 Mohl 1833. 2 Dangeard 1894. 3 Baur 1904. * Nienburg 1908. 5 Maire 1905. 



Fig. 104. Developing asci of Physcia 
ciliaris DC. x 800 (after Baur). 



1 86 REPRODUCTION 

F. Bachmann 1 was able to make important cytological observations in 
her study of Collema pulposum. As regards the vegetative and ascogonial 
nuclei, five or perhaps six chromosomes appeared on the spindle when the 
nucleus divided. In the asci, the usual paired nuclei were present in the 
early stages and did not fuse until the ascus had elongated considerably. 
After fusion the definitive nucleus enlarged with the growth of the ascus 
and did not divide until the ascus had attained full size. The nucleolus was 
large, and usually excentric, and there were at first a number of chromatin 
masses on an irregular spirem. In synapsis the spirem was drawn into a 
compact mass, but after synapsis, "the chromatin is again in the form of 
a knotty spirem." In late prophases the chromosomes, small ovoid bodies, 
were scattered on the spindle; later they were aggregated in the centre, 
and, in the early metaphase, about twelve were counted now split longi- 
tudinally. There were thus twice as many chromosomes in the first division 
in the ascus as in nuclear divisions of the vegetative hyphae. F. Bachmann 
failed to see the second division ; there were at least five chromosomes in the 
third division. 

Considerable importance is given to the number of the chromosomes in 
the successive divisions in the ascus since they are considered to be proof of 
a previous double fusion in the ascogonium and again in the ascus necessi- 
tating, therefore, a double reduction division to arrive at the gametophytic 
or vegetative number of five or six chromosomes in the third division in the 
ascus. There have been too few observations to draw any general conclusions. 

c. DEVELOPMENT OF SPORES. The spore wall begins to form, as in 
Ascomycetes, at the apex of the nucleus with the curving over of the astral 
threads, the nucleus at that stage presenting the figure of a flask the neck 
of which is occupied by the centrosome. The final spore-nucleus, as observed 
by Maire, divides once again in Anaptychia and division is followed by the 
formation of a median septum, the mature spore being two-celled. In 
Peltigera the spore is at first ovoid, but both nucleus and spore gradually elon- 
gate. The fully formed spore is narrowly fusiform and by repeated nuclear 
division and subsequent cross-septation it becomes 4- or even 5-6-celled. 

The spores of lichens are wholly fungoid, and, in many cases, form a 
parallel series with the spores of the Ascomycetes. Markings of the epispore, 
such as reticulations, spines, etc., are rarely present (Solorina spongiosa), 
though thickening of the wall occurs in many species (Pertusariae, etc.), a 
peculiarity which was first pointed out by Mohl 2 who contrasted the spore 
walls with the delicate membranes of other lichen cells. Some spores, 
described as "halonate," have an outer gelatinous covering which probably 
prevents the spore from drying up and thus prolongs the period of possible 
germination. Both asci and spores are, as a rule, more sparingly produced 

1 Bachmann 1913. 2 Mohl 1833. 



LICHEN ASCI AND SPORES 187 

than in fungi; in many instances some or all of the spores in the ascus 
are imperfectly formed, and the full complement is frequently lacking, 
possibly owing to some occurrence of adverse conditions during the long 
slow development of the apothecium. In the larger number of genera and 
species the spores are small bodies, but in some, as for instance in the 
Pertusariae and in some Pyrenocarpeae, they exceed in size all known fungus 
spores. In Varicellaria microsticta, a rare crustaceous lichen of high moun- 
tains, the solitary i -septate spore measures up to 350/4 in length and 1 15 /* in 
width. Most spores contain reserve material in the form of fat, etc., many are 
dark-coloured; Zukal 1 has suggested that the colour may be protective. 

Their ejection from the ascus at maturity is caused by the twofold 
pressure of the paraphyses and the marginal hyphae on the addition of 
moisture. The spores may be shot up at least I cm. from the disc 2 . 

d. SPORE GERMINATION. Meyer 3 was the first who cultivated lichen 
spores and the dendritic formation which he obtained by growing them on 
a smooth surface was undoubtedly the prothallus (or hypothallus) of the 
lichen. Actual germination was however not observed till Holle 4 in 1846 
watched and figured the process as it occurs in Physcia ciliaris. 

Spores divided by transverse septa into two or more cells, as well as 
those that have become "muriform" by transverse and longitudinal septation, 
may germinate from each cell. 

e. MuLTINUCLEATE SPORES. These spores, which are all very large, 
occur in several genera or subgenera: in Lecidea subg. Mycoblastus (Fig. 105), 
Lecanora subg. Ochrolechia and in Pertusariaceae. Tulasne 5 in his experi- 





Fig. 105. Multinucleate spore of Lecidea Fig. 106. Germination of multinucleate 

(Mycoblastus) sanguinaria Ach. x 540 spore of Ochrolechia pallescens Koerb. 

(after Zopf). x 390 (after de Bary). 

ments with germinating spores found that in Lecanora parella (Ochrolechia 

pallescens^} germinating tubes were produced all over the surface of the 

spore (Fig. 106). De Bary 8 verified his observations in that and other species 

and added considerable detail : about twenty-four hours after sowing spores 

of Ochrolechia pallescens, numerous little warts arose on the surface of the 

1 Zukal 1895. 2 Fee 1824. 3 Meyer 1825. 4 Holle 1849. 

* Tulasne 1852. 6 De Bary 1866-1867. 



i88 REPRODUCTION 

spore which gradually grew out into delicate hyphae. All these spores 
contain fat globules and finely granular protoplasm with a very large number 
of minute nuclei; the presence of the latter has been demonstrated by 
Haberlandt 1 and later by Zopf 2 who reckoned about 200 to 300 in the 
spore of Mycoblastus sanguinarius. These nuclei had continued to multiply 
during the ripening of the spore while it was still contained in the ascus 2 . 
Owing to the presence of the large fat globules the plasma is confined to 
an external layer close to the spore wall; the nuclei are embedded in the 
plasma and are connected by strands of protoplasm. The epispore in some 
of these large spores is extremely developed: in some Pertusariae it 
measures 4-5 /* in thickness. 

/ POLARIBILOCULAR SPORES. The most peculiar of all lichen spores 
are those termed polaribilocular signifying a two-celled spore of which the 
median septum has become so thickened that the cell-cavities with their 
contents are relegated to the two poles of the spore, an open canal frequently 
connecting the two cell-spaces (Fig. 107). Other terms have been suggested 
and used by various writers to describe this unusual 
character such as blasteniospore 3 , orculiform 4 and 
placodiomorph 5 or more simply polarilocular. 

The polarilocular colourless spore is found in 
a connected series of lichens crustaceous, foliose 
and fruticose (Placodium, Xanthoria, TeloscMstes). 
In another series with a darker thallus (Rinodina 
and Physcia) the spore is brown-coloured, and the 
Fig. 107. Polarilocular spores, median septum cuts across the plasma-connection. 

a, Xanthoria parietina Th. T , , , ... 

Fr. ; b, Kinodina roboris Th. ^ n other respects the brown spore is similar to the 
M r V ^My^.Pu! 1 *" colourless one and possesses a thickened wall with 

Nyl.; d, Physcia cihans DC. 

x6oo. reduced cell-cavities. 

The method of cell-division in these spores resembles that known as 
" cleavage by constriction," in which the cross wall arises by an ingrowth 
from all sides of the cell; in time the centre is reached and the wall is com- 
plete, or an open pore is left between the divided cells. Cell "cleavage" 
occurs frequently among Thallophytes, though it is unknown among the 
higher plants. Among Algae it is the normal form of cell-division in Clado- 
phora and also in Spirogyra, though in the latter the v/all passes right across 
and.xruts through the connecting plasma threads. Harper 6 found "cleavage 
by constriction " in two instances among fungi : the conidia of Erysiphe and 
the gametes of Sporodinia are cut off by a septum which originates as a 
circular ingrowth of the outer wall, comparable, he considers, with the cell- 
division of Cladophora. 

1 Haberlandt 1887. 2 Zopf iy) ^ a Massalongo 1852. 4 Koerber 1855. 

6 Wainio i. 1890, p. 113. Harper 1899. 




LICHEN ASCI AND SPORES 189 

The development of the thickened wall of polarilocular spores has been 
studied by Hue 1 , who contends however that there is no true septation in 
the colourless spores so long as the central canal remains open. According 
to his observations the wall of the young spore is formed of a thin tegument, 
everywhere equal in thickness, and consisting of concentric layers. This 
tegument becomes continually thicker at the equator of the spore by the 
addition of new layers from the interior, and the protoplasmic contents are 
compressed into a gradually diminishing space. In the end the wall almost 
touches at the centre, and the spore consists of two polar cell-cavities with 
a narrow open passage between. A median line pierced by the canal is 
frequently seen. In a few species there is a second constriction cleavage 
and the spore becomes quadrilocular. 

Hue insists that this spore should be regarded as only one-celled; for 
though the walls may touch at the centre, he says they never coalesce. He 
has unfortunately given no cytological observations as to whether the spore 
is uni- or binucleate. 

In Xanthoria parietina, one of the species with characteristic polari- 
bilocular spores, germination, it would seem, takes place mostly at one end 
only of the spore, though a germinating tube issues at both ends frequently 
enough to suggest that the spore is binucleate and two-celled. The absence 
of germination from one or other of the cells only may probably be due to 
the drain on their small resources. Hue has cited the rarity of such instances 
of double germination in support of his view of the one-celled nature of the 
spore. He instances that out of fifteen spores, Tulasne 2 has figured only 
three that have germinated at each end; Bornet 3 figures one in seven with 
the double germination and Bonnier 4 one in sixteen spores. 

Further evidence is wanted as to the nuclear history of these hyaline 
spores. In the case of the brown spores, which show the same thickening 
of the wall and restricted cell-cavity, though with a distinct median septum, 
nuclear division was observed by Rend Maire 5 before septation in one such 
species, Anaptychia ciliaris. 

II. SECONDARY SPORES 

A. REPRODUCTION BY OIDIA 

In certain conditions of nutrition, fungal hyphae break up into separate 
cells, each of which functions as a reproductive conidium or oidhim, which 
on germination forms new hyphae. Neubner 6 has demonstrated a similar 
process in the hyphae of the Caliciaceae and compares it with the oidial 
formation described by Brefeld 7 in the Basidiomycetes. 

1 Hue 191 1 2 . 2 Tulasne 1852. 3 Bornet 1873. 4 Bonnier i889 2 . 

5 Maire 1905. 6 Neubner 1893. 7 Brefeld 1889. 



REPRODUCTION 



The thallus of this family of lichens is granular or furfuraceous ; it never 
goes beyond the Lepra stage of development 1 . In some species it is scanty, 
in others it is abundant and spreads over large areas of the trunks of old 
trees. It is only when growth is especially luxuriant that oidia are formed. 
Neubner was able to recognize the oidial condition by the more opaque 
appearance of the granules, and under the microscope he observed the 
hyphae surrounding the gonidia gradually fall away and break up into 
minute cylindrical cells somewhat like spermatia in size and form. There 
was no question of abnormal or unhealthy conditions, as the oidia were 
formed in a freely fruiting thallus. 

The gonidia associated with the oidial hyphae also showed unusual 
vitality and active division took place as they were set free by the breaking 
up of the encircling hyphae. The germination of the oidia provides an 
abundance of hyphal filaments for the rapidly increasing algal cells, and 
there follows a widespread development of the lichen thallus. 

Oidial formation has not been observed in any other family of lichens. 

B. REPRODUCTION BY CONIDIA 

a. INSTANCES OF CONIDIAL FORMATION. It is remarkable that the 
type of asexual reproduction so abundantly represented in fungi by the large 
and varied group of the Hyphomycetes is prac- 
tically absent in lichens. An exception is to be 
found in a minute gelatinous lichen that grows on 
soil. It was discovered by Bornet 2 and called by 
him Arnoldia (Physmd) minutula. From the thallus 
rise up simple or sparingly branched colourless 
conidiophores which bear at the tips globose brown 
conidia(Fig. 108). Bornet 3 obtained these conidia 
by keeping very thin sections of the thallus in a 
drop of water 2 . 

Yet another instance of conidial growth is given 
by Steiner 4 . He had observed that the apothecia 
on plants of Caloplaca aurantia var. callopisma 
Stein, differed from those of normal appearance 
in the warted unevenness of the disc and also in 
being more swollen and convex, the thalline margin 
being almost obliterated. He found, on micro- 
scopical examination, that the hymenium was 
occupied by paraphyses and by occasional asci, 
the latter seldom containing spores, and being 

2 Bornet 1873. 




Conidia developed 
om thallus of Arnoldia mi- 



nutula Born. 
Bornet). 

See p. 143. 



x 950 (after 



3 Bornet's observations have not been repeated, and it is possible that he may have been dealing 
with a parasitic hyphomycetous fungus. 4 Steiner 1901. 



LICHEN ASCI AND SPORES 191 

usually more or less collapsed. The component parts of the apothecium 
were entirely normal and healthy, but the paraphyses and the few asci were 
crushed aside by the intrusion of numerous slender unbranched septate 
conidiophores. Several of these might spring from one base and the hypha 
from which they originated could be traced some distance into the ascogenous 
layer, though a connection with that cell-system could not be demonstrated. 
While still embedded in the hymenium, an ellipsoid or obovate swelling 
began to form at the apex of the conidiophore; it became separated from 
the stalk by a septum and later divided into a two-celled conidium. 
The conidiophore increased in length by intercalary growth and finally 
emerged above the disc; the mature conidium was pyriform and measured 
1 5-20 /z, x 9-11 yu, 

Steiner regarded these conidia as entirely abnormal; pycnidia with 
stylospores are unknown in the genus and they were not, he alleges, the 
product of any parasitic growth. 

b. COMPARISON WITH HYPHOMYCETES. The conidial form of fructi- 
fication in fungi, known as a Hyphomycete, is generally a stage in the life- 
cycle of some Ascomycete; it represents the rapid summer form of asexual 
reproduction. The ascospore of the resting fruit-form in many species ger- 
minates on any suitable matrix and may at once produce conidiophores and 
conidia, which in turn germinate, and either continue the conidial generation 
or proceed to the formation of the perfect fruiting form with asci and asco- 
spores. 

Such a form of transient reproduction is almost impossible in lichens, as 
the hypna produced by the germinating lichen ascospore has little vitality 
without the algal symbiont. In natural conditions development practically 
ceases in the absence of symbiosis. When union between the symbionts 
takes place, and growth becomes active, thallus construction at once com- 
mences. But in certain conditions of shade and moisture, only the rudiments 
of a lichen thallus are formed, known as a leprose or sorediose condition. 
Soredia also arise in the normal life of many lichens. As the individual 
granules or soredia may each give rise to a complete lichen plant, they may 
well be considered as replacing the lost conidial fructification. 

C. CAMPYLIDIUM AND ORTHIDIUM 

Mu'ller 1 has described under the name Campy lidium a supposed new type 
of asexual fructification which he found on the thallus of tropical species of 
Gyalecta, Lofadium, etc., and which he considered analogous to pycnidia and 
spermogonia. Wainio 2 has however recognized the cup-like structure as a 
fungus, CypJiella aeruginascens Karst, which grows on the bark of trees and 
occasionally is parasitic on the crustaceous thallus of lichens. Wainio has 
1 Miiller 1881. 2 Wainio 1890, n. p. 27. 



1 9 2 



REPRODUCTION 



also identified the plant, Lecidea irregnlaris, first described by Fe'e 1 , as also 
synonymous with the fungus. 

Another name Orthidium was proposed by M tiller 2 for a type of fructi- 
fication he found in Brazil which he contrasts or associates with Campylidium. 
It has an open marginate disc with sporophores bearing acrogenous spores. 
He found it growing in connection with a thin lichen thallus on leaves and 
considered it to be a form of lichen reproduction. Possibly Orthidium is 
also a Cyphella. 

III. SPERMOGONIA OR PYCNIDIA 

A. HISTORICAL ACCOUNT OF SPERMOGONIA 

The name spermogonium was given by Tulasne 3 to the " punctiform 
conceptacles " that are so plentifully produced on many lichen thalli, on the 
assumption that they were the male organs of the plant, and that the spore- 
like bodies borne in them were non-motile male cells or spermatia. 

The first record of their association with lichens was made by Dillenius l , 
who indicates the presence of black tubercles on the thallus of Physcia 
dliaris. He figures them also on several species of Cladonia, on Ramalina 
and on Dermatocarpon, but without any suggestion as to their function. 
Hed wig's 5 study of the reproductive organs of the Linnaean Cryptogams 
included lichens. He examined Physcia dliaris, a species that not only is 
quite common but is generally found in a fruiting condition and with very 
prominent spermogonia,and has been therefore a favourite lichen for purposes 
of examination and study. Hedwig describes and figures not only ^he apo- 
thecia but also those other bodies which he designates as "punctula mascula," 
or again as " puncta floris masculi." In his later work he gives a drawing 
of Lichen (Gyrophord) proboscideus, with two of the spermogonia in section. 

Acharius 6 included them among the lichen structures which he called 
" cephalodia": he described them as very minute tubercles rising up from 
the substance of the thallus and projecting somewhat above it. He also 
figures a section through two " cephalodia " of Physda dliaris. Fries 7 looked 
on them as being mostly " anamorphoses of apothecia, the presence of 
abortive fruits transforming the angiocarpous lichen to the appearance of a 
gymnocarpous form." Wallroth 8 assigned the small black fruits to the com- 
prehensive fungus genus Sphaeria or classified lichens bearing spermogonia 
only as distinct genera and species (Pyrenothea and Thrornbiuni). Later 
students of lichens Schaerer 9 , Flotow 10 , and others accepted Wallroth's 
interpretation of their relation to the thallus, or they ignored them altogether 
in their descriptions of species. 

1 Fee 1873. 2 Miiller 1890. 3 Tulasne 1851. * Dillenius 1741. *> Hedwig 1784 and 1789. 
6 Acharius 1 8 10. 7 Fries 1831. 8 Wailroth 1825. 9 Schaerer 1823-1842. 10 Flotow 1850. 



SPERMOGONIA 193 

B. SPERMOGONIA AS MALE ORGANS 

Interest in these minute "tubercles" and their enclosed "corpuscles" 
was revived by Itzigsohn 1 who examined them with an improved microscope. 
He macerated in water during a few days that part of the thallus on which 
they were developed, and, at the end of the time, discovered that the 
solution contained large numbers of motile bodies which he naturally took 
to be the corpuscles from the broken down tubercles. He claimed to have 
established their function as male motile cells or spermatozoa. The discovery 
seemed not only to prove their sexual nature, but to link up the reproduction 
of lichens with that of the higher cryptogams. The tubercles in which the 
" spermatozoa " were produced he designated as antheridia. More prolonged 
maceration of the tissue to the very verge of decay yielded still larger numbers 
of the " spermatozoa " which we now recognize to have been motile bacilli. 

Tulasne 2 next took up the subject, and failing to find the motile cells, 
he wrongly insisted that Itzigsohn had been misled by mere Brownian 
movement, but at the same time he accepted the theory that the minute 
conceptacles were spermogonia or male organs of lichens. He also pointed 
out that their constant occurrence on the thallus of practically every species 
of lichen, and their definite form, though with considerable variation, rendered 
it impossible to regard them as accidental or of no importance to the life of 
the plant. He compared them with fungal pycnidia such as Phyllosticta or 
Septoria which outwardly they resembled, but whereas the pycnidial spores 
germinated freely, the spermatia of the spermogonia, as far as his experience 
went, were incapable of germination. 

C. OCCURRENCE AND DISTRIBUTION 

a. RELATION TO THALLUS AND APOTHECIA. We owe to Tulasne 3 the 
first comparative study of lichen spermogonia. He described not only 
their outward form, but their minute structure, in a considerable number 
of representative species. A few years later Lindsay 4 published a memoir 
dealing with the spermogonia of the larger foliose and fruticose lichens, and, 
in a second paper, he embodied the results of his study of an equally ex- 
tensive selection of crustaceous species. Lindsay's work is unfortunately 
somewhat damaged by faulty determination of the lichens he examined, and 
by lack of the necessary discrimination between one thallus and another of 
associated and intermingled species. Both memoirs contain, however, much 
valuable information as to the forms of spermogonia, with their spermatio- 
phores and spermatia, and as to their distribution over the lichen thallus. 

Though spermogonia are mostly found associated with apothecia, yet 

1 Itzigsohn 1850. - Tulasne 1851. 3 Tulasne 1852. 4 Lindsay 1859 and 1872. 

S. L. 13 



194 



REPRODUCTION 



in some lichens, such as Cerania ( Thamnolia) vermicularis, they are the only 
sporiferous organs known. Not unfrequently crustaceous thalli bear sper- 
mogonia only, and in some Cladoniae, more especially in ascyphous species, 
spermogonia are produced abundantly at the tips of the podetial branches 
(Fig. 109), while apothecia are exceedingly rare. Usually they occur in 
scattered or crowded groups, more rarely they are solitarj'. Very often they 
are developed and the contents dispersed before the apothecia reach the 
surface of the thallus; hence the difficulty in relating these organisms, since 
the mature apothecium is mostly of extreme importance in determining the 
species. 





Fig. 109. C/adomafurcataSchrad. Branched 
podetium with spermogonia at the tips 
. (after Krabbe). 



Fig. no. Physcia hispida Tuckerm. Ciliate 
frond, a, spermogonia ; 6, apothecia. x ca. 5 
(after Lindsay). 



In a very large number of lichens, both crustaceous and foliose, the 
spermogonia are scattered over the entire thallus (Fig. 1 10). covering it more 
or less thickly with minute black dots, as in Parmelia conspersa. In other 
instances, they are to some extent confined to the peripheral areas as in 
Parmelia physodes ; or they occur on the extreme edge of the thallus as in 
the crustaceous species Lecanora glaucoma (sordidd). In Pyrenula nitida 
they grow on the marginal hypothallus, usually on the dark line of demar- 
cation between two thalli. 

They tend to congregate on, and indeed are practically restricted to the 



SPERMOGONIA 



195 



better lighted portions of the thallus. On the fronds of foliose forms, they 
appear, for instance, on the swollen pustules of Umbilicaria pustulata, while 
in Lobaria pulmonaria, they are mostly lodged in the ridges that surround 
the depressions in the thallus. In Parmelia conspersa, Urceolaria (Diplo- 
schistes) scruposa and some others, they occasionally invade the margins of 
the apothecium or even the apothecial disc as in Lichina. Forssell 1 found 
that a spermogonium had developed among cells of Gloeocapsa that covered 
the disc of a spent apothecium of Pyrenopsis haematopis. 

In fruticose lichens such as Usnea, Ramalina, etc. they occur near the 
apex of the fronds, and in Cladonia they occupy the tips of the ascyphous 
podetia or the margins of the scyphi. In some Cladoniae, however, spermo- 
gonia are produced on the basal squamules, more rarely on the squamules 
that clothe the podetia. 

b. FORM AND SIZE. Spermogonia are specifically constant in form, the 
same type being found on the same lichen species all over the globe. The 
larger number are entirely immersed and are ovoid or roundish (Fig. 1 1 1 A) 
or occasionally somewhat flattened bodies (Nephromium laevigatum),ov again, 
but more rarely, they are irregular in outline with an infolding of the walls 
that gives the interior a chambered form (Fig. 1 1 1 B) (Lichina pygmaed) ; but 
all of these are only visible as minute points on the thallus. 





B 

Fig. in. Immersed spermogonia. A, globose in Parmelia 
acetabulum Dub. x 600 ; B, with infolded walls in Lecidea 
(Psora) testacea Ach. x 144 (after Gliick). 

A second series, also immersed, are borne in small protuberances of the 
thallus. These very prominent forms are rarely found in crustaceous lichens, 
but they are characteristic of such well-known species as Ramalina fraxinea, 
Xanthoria parietina, Ricasolia ampltssima, Baeomyces roseus, etc. Other sper- 
mogonia project slightly above the level of the thallus, as in Cladonia papillaria 
and Lecidea lurida; while in a few instances they are practically free, these 
last strikingly exemplified in Cetraria islandica where they occupy the 
small projections or cilia (Fig. 112) that fringe the margins of the lobes; they 
are free also in most species of Cladonia. 

1 Forssell 1885. 

132 



REPRODUCTION 



In size they vary from such minute bodies as those in Parmelia exasperata 
which measure 25-35 p, in diam., up to nearly I mm. in Lobaria laetevirens. 

As a rule, they range from about 150/4 
to 400 fj, across the widest part, and are 
generally rather longer than broad. They 
open above by a small slit or pore called 
the ostiole about 20 yu, to I oo /x wide which 
is frequently dark in colour. In one in- 
stance, in Icmadophila aeruginosa, Nien- 
burg 1 has described a spermogonium with 
a wide opening, the spermatiophores 
being massed in palisade formation along 
the bottom of a cup-like structure. 

c. COLOUR OF SPERMOGONIA. Though 
usually the ostiole is visible as a darker 
point than the surrounding tissue, sper- 
mogonia are often difficult to locate un- 
less the thallus is first wetted, when they become visible to slight magnification. 
They appear as black points in many Parmeliae,Physciae,Roccellae, etc., though 
even in these cases they are often brown when moistened. They are dis- 
tinctly brown in some Cladoniae, in Nephromium, and in some Physciae\ 
orange-red or yellow in Placodium and concolorous with the thallus in 
Usnea, Ramalina, Stereocaulon, etc. 




Fig 



j. 112. rree spermogonia in spmous 
cilia of Cetraria islandica Ach. A, part 
of frond; B, cilia, x 10. 



D. STRUCTURE 

a. ORIGIN AND GROWTH. The spermogonia (or pycnidia) of lichens 
when mature are more or less hollow structures provided with a distinct 
wall or " perithecium," sometimes only one cell thick and then not easily de- 
monstrable, as in Physcia speciosa, Opegrapha vulgata, Pyrenula nitida, etc. 
More generally the " perithecium " is composed of a layer of several cells 
with stoutish walls which are sometimes colourless, but usually some shade 
of yellow to dark-brown, with a darker ostiole. The latter, a small slit or 
pore, arises by the breaking down of some of the cells at the apex. After 
the expulsion of the spermatia, a new tissue is formed which completely 
blocks up the empty spermogonium. In filamentous lichens such as Usnea 
a dangerous local weakening of the thallus is thus avoided. 

Spermogonia originate from hyphae in or near the gonidial zone. The 
earliest stages have not been seen, but Moller 2 noted as the first recogniz- 
able appearance or primordium of the "pycnidia" in cultures of Calicium 
trachelinum a ball or coil of delicate yellowish-coloured hyphae. At a more 

1 Nienburg roo8. 2 Moller 1887. 



SPERMOGONIA 197 

advanced stage the sporophores (or spermatiophores) could be traced as 
outgrowths from the peripheral hyphae, directed in palisade formation 
towards the centre of the hyphal coil about 20-30 (j. long and very slender 
and colourless. They begin to bud off spermatia almost immediately, as it 
has been found that these are present in abundance while the developing 
spermogonium is still wholly immersed in the thallus. Meanwhile there is 
gradually formed on the outside a layer of plectenchyma which forms 
the wall. Additional spermatiophores arise from the wall tissue and push 
their way inwards between the ranks of the first formed series. The sper- 
mogonium slowly enlarges and stretches and as the spermatiophores do not 
grow any longer a central hollow arises which becomes packed with sper- 
matia (or spores) before the ostiole is open. 

A somewhat similar process of development is described by Sturgis 1 in 
the spermogonia of Ricasolia amplissima, in which species the primordium 
arises by a profuse branching of the medullary hyphae in certain areas close 
to the gonidial zone. The cells of these branching hyphae are filled with oily 
matter and gradually they build up a dense, somewhat cylindrical body 
which narrows above to a neck-like form. The growth is upwards through 
the gonidial layer, and the structure widens to a more spherical outline. It 
finally reaches the outer cortex when some of the apical cell membranes 
are absorbed and a minute pore is formed. The central part becomes hollow, 
also by absorption, and the space thus left is lined and almost filled with 
multicellular branches of the hyphae forming the wall; from the cells of 
this new tissue the spermatia are abstricted. 

b. FORMS AND TYPES OF SPERMATIOPHORES. The variations in form of 
the fertile hyphae in the spermogonium were first pointed out by Nylander- 
who described them as sterigmata 3 . He considered the differences in 
branching, etc. as of high diagnostic value, dividing them into two groups: 
simple "sterigmata" (or spermatiophores), with non-septate hyphae, and 
arthrosterigmata, with jointed or septate hyphae. 

Simple " sterigmata " comprise those in which the spore or spermatium is 
borne at the end of a secondary branch or sterigma, the latter having arisen 
from a cell of the upright spermatiophore or from a simple basal cell. The 
arthrosterigmata consist of " short cells almost as broad as they are long, 
much pressed together, and appearing almost agglutinate especially toward 
the base; they fill almost the whole cavity of the spermogonium." The 
arthrosterigmata may grow out into the centre of the cavity as a single 
cell-row, as a loose branching network, or, as in Endocarpon, they may form 

1 Sturgis 1890. 2 Nylander. 1858, pp. 34, 35. 

3 Nylander, Crombie and others apply the term "sterigma" to the whole spermatiophore. In 
the more usual restricted sense, it refers only to the short process from which the spermatium is 
abstricted. 



REPRODUCTION 



a tissue filling the whole interior. Each cell of this tissue that borders on 
a cavity may bud off a spermatium either directly or from the end of a 
short process. 

The most important contributions on the subject of spermogonia in 
recent years are those of Gliick 1 and Steiner 2 . Gliick, who insisted on the 











7 ig- 1 13 A - Types of lichen " sporophores " and pycnidiospores. i , 
Pdtigera rufescens fioffm. x 910; 2, Lecidea (Psora) testacea Ach. 
x 1200; 3, Cladonia cariosa Spreng. x 1000; 4, Pyrenula nitida 
Ach. x 1130; 5, Parmelia trtstis Nyl. x 700; 6, Lobaria pulmo- 
naria Hoffm. x jooo (after Gliick). 

1 Gluck 1899., 2 Steiner 1901. 



SPERMOGONIA 199 

"pycnidial" non-sexual character of the organs, recognized eight types of 
"sporophores" differing in the complexity of their branching or in the form 
of the "spores" (Fig. 1 13 A): 

1. The Peltigera type: the sporophores consist of a basal cell bearing 
one or more long sterigmata and rather stoutish ellipsoid spores. (These 
are true pycnidia.) 

2. The Psora type: a more elongate simple sporophore with sterigmata 
and oblong spores. 

3. The Cladonia type: a branching sporophore, each branch with sterig- 
mata and oblong spores. 

4. The Squamaria type (called by Gliick Placodiuni) : also a branching 
sporophore but with long sickle-like bent spores. 

5. The Parmelia type: a more complicated system of branching and 
anastomosing of the sporophores, with oblong spores. 

6. and 7. The Sticta and Physcia types: in both of these the sporo- 
phores are multiseptate; they consist of a series of radiately arranged 
hyphae rising from a basal tissue all round the pycnidium. They anasto- 
mose to form a network and bud off " spermatia " from the free cells or 
rather from minute sterigmata. In the Physcia type there is more general 
anastomosis of the sporophores and frequently masses of sterile cells along 
with the fertile members occupy the centre of the pycnidium. The sper- 
matia of these and the following Endocarpon type are short cylindrical 
bodies (Fig.- 1138). 





7 

Fig. 1136. 7, Physcia ciliaris DC. x 600; 8, Endocarpon sp. x 600 
(after Gliick). 

8. Endocarpon type: the pycnidium is filled by a tissue of short broad 
cells, with irregular hollow spaces lined by fertile cells similar to those of 
the Sticta and Physcia types. 



200 REPRODUCTION 

The three last named types of sporophores represent Nylander's section 
of arthrosterigmata. Steiner has followed Nylander in also arranging the 
various forms into two leading groups. The first, characterized by the 
secondary branch or "sterigma," he designates "exobasidial"; the second, 
comprising the three last types in which the spores are borne directly on 
the cells of the sporophore or on very short processes, he describes as " endo- 
basidial." Steiner also introduces a new term, fulcrum > for the sporophore. 
The pycnidia in which these different sporophores occur are not, as a 
rule, characteristic of one family. Peltigera type is found only in one family 
and the Cladonia type is fairly constant in Cladoniae, but "Psora" pycnidia 
are found on very varying lichens among the Lecideaceae, Verrucariaceae 
and others. The Squamaria type with long bent spores is found not only in 
Squamaria (Gliick's Placodium) but also in Lecidea, Roccella, Pyrenula, etc. 
Parmelia type is characteristic of many Parmeliae and also of species of 
Evernia, Alectoria, Platysma and Cetraria. The Sticta type occurs in Gyro- 
phora, Umbilicaria, Nephromium and Lecanora as well as in Sticta and in one 
species at least of Collema. To the Physcia type belong the pycnidia of most 
Physciaceae and of various Parmeliae, and to the closely related Rndocarpon 
type the pycnidia of Endocarpon and of Xanthoria parietina. 

c. PERIPHYSES AND STERILE FILAMENTS. In a few species, Roccella 
tinctoria,Pertusariaglobulifera,&\ic.,shor\. one-celled sterile hyphae are formed 
within the spermogonium near the ostiole, towards which they converge. 
They correspond to the periphyses in the peri- 
thecia of some Pyrenolichens, Verrucaria, etc. 
(described by Gibelli 1 as spermatiophores); they 
are also present in some of the Pyrenomycetes 
(Sordaria, etc.), and in many cases replace the 
paraphyses in function when these have broken 
down. Sterile hyphae also occur, towards the base, 
mingled with the fertile spermatiophores (Fig. 
4 . Sterile filaments in 114). These latter were first described and figured 
SS5ST mLh'mfgnified b 7 Tulasne 2 in the spermogonia of Ramalina 
(after Lindsay). fraxinea as stoutish branching filaments, rising 

from the same base as the spermatiophores but much longer, and frequently 
anastomosing with each other. They have been noted also in Usnea bar- 
bata and in several species of Parmelia, and have been compared by Ny- 
lander 3 to paraphyses. They are usually colourless, but, in the Parmeliae, 
are often brownish and thus easily distinguished from the spermatio- 
phores. It has been stated that these filaments are sometimes fertile. Similar 
sterile hyphae have been recorded in the pycnidia of fungi, in Sporocladus 
(Hendersonia) lichenicola (Sphaeropsideae) by Corda 4 who described them as 
1 Gibelli 1866. 2 Tulasne 1852. 3 Nylander 1858. 4 Corda 1839. 




SPERMOGONIA 201 

paraphyses, and also in Steganosporium cellulosum (Melanconieae). These 
observations have been confirmed by Allescher 1 in his recent work on Fungi 
Imperfecti. Keiszler 2 has described a P/wma-\ike pycnidium parasitic on 
the leprose thallus of Haematomma elatinum. It contains short slender 
sporophores and, mixed with these, long branched sterile hyphae which 
reach to the ostiole and evidently function as paraphyses, though Keiszler 
suggests that they may be a second form of sporophore that has become 
sterile. On account of their presence he placed the fungus in a new genus 
L icJienophoma. 

E. SPERMATIA OR PYCNIDIOSPORES 

a. ORIGIN AND FORM OF SPERMATIA. Lichen spermatia arise at the 
tips of the sterigmata either through simple abstriction or by budding. In 
the former case as in the Squamaria type a delicate cross-wall is formed 
by which the spermatium is separated off. When they arise by budding, 
there is first a small clavate sac r like swelling of the end of the short process or 
sterigma which gradually grows out into a spermatium on a very narrow base. 
This latter formation occurs in the Sticta, Physcia and Endocarpon types. 

Ny lander 3 has distinguished the following forms of spermatia: 

1. Ob-clavate, the ^road end attached to the sterigma as in Usneae, 
Cetraria glauca and C. juniperina. 

2. Acicular and minute but slightly swollen at each end, somewhat 
dumb-bell like, in Cetraria nivalis, C. cucullata, Alectoria, Evernia and some 
Parmeliae, frequently borne on "arthrosterigmata." 

3. Acicular, cylindrical and straight, the most common form ; these occur 
in most of the Lecanorae, Cladoniae, Lecideae, Graphideae, Pyrenocarpeae 
and occasionally they are budded off from arthrosterigmata. 

4. Acicular, cylindrical, bent; sometimes these are very long, measuring 
up to 40 //.; they are found in various Lecideae, Lecanorae, Graphideae, 
Pyrenocarpeae, and also in Roccella, Pilophorus and species of Stereocaulon. 

5. Ellipsoid or oblong and generally very minute; they are borne on 
simple sterigmata and are characteristic of the genera Calicium, Chaenotheca, 
Lichina,Ephebe,ofi\\e. small genus Glypholecia and of a few species tfLecanora 
and Lecidea. 

In many instances there is more or less variation of form and of size in 
the species or even in the individual. There are no spherical spermatia. 

b. SIZE AND STRUCTURE. The shortest spermatia in any of our British 
lichens are those of Lichina pygmaea which are about i'4/A in length and 
the longest are those of Lecanora crassa which measure up to 39 ft. In width 
they vary from about O'5/tt to 2/z. The mature spermogonium is filled with 

1 Allescher 1901-3. 2 Keiszler 1911. 3 Nylander 1858, p. 37- 



202 REPRODUCTION 

spermatia and, generally, with a mass of mucilage that swells with moisture 
and secures their expulsion. 

The spermatia of lichens are colourless and are provided with a cell-wall 
and a nucleus. The presence of a nucleus was demonstrated by Holier 1 in 
the spermatia of Calicium parietinum, Opegrapha atra, Collema micropkyllum, 
C.pulposum and C. Hildenbrandii, and by Istvanffi 2 in those of Buellia puncti- 
formis (B. myriocarpa), Opegrapha subsiderella, Collema Hildenbrandii, Cali- 
cium trachelinum,Pertusaria communis andArt&oma communis (A. astroided). 
Istvanffi made use of fresh material, fixing the spermatia with osmic acid, 
and in all of these very minute bodies he demonstrated the presence of a 
nucleus which stained readily with haematoxylin and which he has figured 
in the spermatia of Buellia punctiformis as an extremely small dot-like 
structure in the centre of the cell. On germination, as in the cell-multi- 
plication of other plants, the nucleus leads the way. Germination is preceded 
by nuclear division, and each new hyphal cell of the growing mycelium 
receives a nucleus. 

c. GERMINATION OF SPERMATIA (pycnidiospores). The strongest argu- 
ment in favour of regarding the spermatia of lichens as male cells had always 
been the impossibility of inducing their germination. That difficulty had at 
length been overcome by Moller 1 who cultivated them in artificial solutions, 
and by that means obtained germination in nine different lichen species. 
He therefore rejected the commonly employed terms spermatia and spermo- 
gonia and substituted pycnoconidium and pycnidia. Pycnidiospore has 
been however preferred as more in accordance with modern fungal termi- 
nology. His first experiment was with the "spermatia" of Buellia punctiformis 
(B. myriocarpa) which measure about 8-10/1. in length and about 3 ^ in 
width, and are borne directly on the septate spermatiophores (arthrosterig- 
mata). In a culture drop, the spore had swelled to about double its size by 
the second or third day, and germination had taken place at both ends, the 
membrane of the spore being continuous with that of the germinating tube. 
In a short time cross septa were formed in the hyphae which at first were 
very close to each other. While apical growth advanced these first formed 
cells increased in width to twice the original size and, in consequence, became 
slightly constricted at the septa. In fourteen days a circular patch of my- 
celium had been formed about 280/1 in diameter. The development exactly 
resembled that obtained from the ascospores of the same species grown in the 
absence of gonidia. The largest thallus obtained in either case was about 
2mm. in diameter after three months' growth. The older hyphae had a 
tendency to become brownish in colour; those at the periphery remained 
colourless. In Opegrapha subsiderella the development, though equally 

1 Moller 1887. 2 Istvanffi 1895. 



SPERMOGONIA 203 

successful, was very much slower. The pycnidiospores (or spermatia) have 
the form of minute bent rods measuring 57 /t x 1-5 /i. Each end of the spore 
produced slender hyphae about the fifth or sixth day after sowing. In four 
weeks, the whole length of the filament with the spore in the middle was 
300/1. In four months a patch of mycelium was formed 2 mm. in diameter. 
Growth was even more sluggish with the pycnidiospores of Opegrapha atra. 
In that species they are rod-shaped and 5-6/4 long. Germination took place on 
the fifth or sixth day and in fourteen days a germination tube was produced 
about five times the length of the spore. In four weeks the first branching 
was noticed and was followed by a second branching in the seventh week. 
In three months the mycelial growth measured 200-300/4 across. 

Germination was also observed in a species of Arthonia, the spores of 
which had begun to grow while still in the pycnidium. The most complete 
results were obtained in species of Calicium : in C. parietinum the spores, 
which are ovoid, slightly bent, and brownish in colour, swelled to an almost 
globose shape and then germinated by a minute point at the junction of spore 
and sterigma, and also at the opposite end; very rarely a third germinating 
tube was formed. Growth was fairly rapid, so that in four weeks there was 
a loose felt of mycelium measuring about 2 cm. x i cm. and I mm. in depth. 
Parallel cultures were carried out with the ascospores and the results in both 
cases were the same; in five or six weeks small black points appeared, which 
gradually developed to pycnidia with mature pycnidiospores from which 
further cultures were obtained. 

On C. trachelinum, which has a thin greyish-white thallus spreading over 
old trunks of trees, the pycnidia are usually abundant. Lindsay had noted 
two different kinds and his observation was confirmed by Moller. The 
spores in one pycnidium are ovoid, measuring 2-5-3 /u, x J'S" 2 ^; m tne 
other rarer form, they are rod-shaped and 5~7/t long. In the artificial 
cultures they both swelled, the rod-like spores to double their width before 
germination, and sometimes several tubes were put forth. Growth was slow, 
but of exactly the same kind from these two types of spores as from the 
ascospores. At the end of the second month pycnidia appeared on all the 
cultures, in each case producing the ovoid type of spore. 

In a second paper Moller 1 recorded the partially successful germination 
of the "spermatia" of Collema (Leptogium) microphyllum, the species in which 
Stahl had demonstrated sexual reproduction. Growth was extraordinarily 
slow : after a month in the culture solution the first swelling of the sper- 
matium prior to germination took place, and some time later small processes 
were formed in two or three directions. In the fourth month a branched 
filament was formed. 

Moller's experiments with ascospores and pycnidiospores were primarily 

1 Moller 1888. 



204 REPRODUCTION 

undertaken to prove that the lichen hyphae were purely fungal and parasitic 
on the algae. A series of cultures were made by Hedlund 1 in order to 
demonstrate that the pycnidiospores were asexual reproductive bodies ; 
they were grown in association with the lichen alga and their germination 
was followed up to the subsequent formation of a lichen thallus. 

d. VARIATION IN PYCNIDIA. On the thallus of Catillaria denigrata 
(Biatorina synothed) Hedlund found that there were constantly present two 
types of pycnidia: the one with short slightly bent spores 4-8 yu, x 1*5 //,, the 
other with much longer bent spores 10-20 ft x 1-5 p; there. were numerous 
transition forms between the two kinds of spores. Germination took place 
by the prolongation of the spore ; the hypha produced became septate and 
branches were soon formed. Hedlund found that frequently germination 
had already begun in the spores expelled from the spermogonium. In newly 
formed thalline areolae it was possible to trace back the mycelium to innu- 
merable germinating spores of both types, long and short. 

Lindsay had recorded more than one form of spermogonium on the 
same lichen thallus, the spermatia varying considerably in size; but he was 
most probably dealing with the mixed growth of more than one species. 
The observations of Moller and Hedlund on this point are more exact, but 
the limits of variation would very well include the two forms found by 
Moller in Calicium tradielinum ; and in the different pycnidia of Catillaria 
denigrata Hedlund not only observed transition stages between the two 
kinds of spores, but the longer pycnidiospores, as he himself allows, indicated 
the elongation prior to germination : there is no good evidence of more than 
one form in any species. 

F. PYCNIDIA WITH MACROSPORES 

Tulasne 2 records the presence on the lichen thallus of "pycnidia" as 
well as of "spermogonia"; the former producing stylospores, larger bodies 
than spermatia, occasionally septate and containing oil-drops or guttulae. 
These spores are pyriform or ovoid in shape and are always borne at the 
tips of simple sporophores. He compared the pycnidia with the fungus 
genera Cytospora, Septoria, etc. As a rule they occur on lichens with a 
poorly developed thallus, on some species of Lecanora, Lecanactis, Cali- 
cium, Porina, in the family Strigulaceae and in Peltigera. 

There is no morphological difference between pycnidia and spermogonia 
except that the spermatia of the latter are narrower ; but the difference is 
so slight that, as Steiner has pointed out, these organs found on Lecanora 
piniperda, L. Sambuci and L. effusa have been described at one time as 
containing microconidia (spermatia), at another macroconidia (stylospores). 
1 Hedlund 1892. 2 Tulasne 1852. 



SPERMOGONIA 205 

He also regards as macrospores those of the pycnidia of Calicium tra- 
chelinum which Moller was able to germinate so successfully, and all the 
more so as they were brownish in colour, true microspores or spermatia 
being colourless. 

Miiller 1 has recorded some observations on the pycnidia and stylospores 
of the Strigulaceae, a family of tropical lichens inhabiting the leaves of 
the higher plants. On the thallus of Strigula elegans var. tremula from 
Madagascar and from India, he found pycnidia with stylospores of abnormal 
dimensions measuring 18-26/4 in length and 3 /A in width, and with I to 7 
cross septa. In Strigula complanata var. genuina the stylospores were 2-8- 
septate and varied from 7-65 /* in length, some of the spores being thus 
ten times longer than others, while the width remained the same. Miiller 
considers that in these cases the stylospore has already grown to a septate 
hypha while in the pycnidium. As in the pycnidiospores, described later 
by Hedlund, the spores had germinated by increase in length followed by 
septation. 

The spermogonia of Strigula, which are exactly similar to the pycnidia 
in size and structure, produce spermatia, measuring about 3/4 x 2/*, and it is 
suggested by Miiller that the stylospores may represent merely an advanced 
stage of development of these spermatia. Both organs were constantly 
associated on the same thallus ; but whereas the spermogonia were abundant 
on the younger part of the thallus at the periphery, they were almost 
entirely replaced by pycnidia on the older portions near the centre, only 
a very few spermogonia (presumably younger pycnidial stages) being found 
in that region. 

Lindsay 2 has described a great many different lichen pycnidia, but in 
many instances he must have been dealing with species of the "Fungi imper- 
fecti" that were growing in association with the scattered granules of 
crustaceous lichens. There are many fungi Discomycetes and Pyreno- 
mycetes parasitic on lichen thalli, and he has, in some cases, undoubtedly 
been describing their secondary pycnidial form of fruit, which indeed may 
appear far more frequently than the more perfect ascigerous form, and might 
easily be mistaken for the pycnidial fructification of the lichen. 

G. GENERAL SURVEY 

a. SEXUAL OR ASEXUAL. It has been necessary to give the preceding 
detailed account of these various structures pycnidia or spermogonia in 
view of the extreme importance attached to them as the possible male 
organs of the lichen plant, and, in giving the results obtained by different 
workers, the terminology employed by each one has been adopted as far as 

1 Miiller 1885. 2 Lindsay 1859 and 1872. 



2o6 REPRODUCTION 

possible: those who consider them to be sexual structures call them spermo- 
gonia ; those who refuse to accept that view write of them as pycnidia. 

Tulasne, Nylander and others unhesitatingly accepted them as male 
organs without any knowledge of the female cell or of any method of ferti- 
lization. Stahl's discovery of the trichogyne seemed to settle the whole 
question ; but though he had evidence of copulation between the spermatium 
and the receptive cell or trichogyne he had no real record of any sexual 
process. 

Many modern lichenologists reject the view that they are sexual; they 
regard them as secondary organs of fructification analogous to the pycnidia 
so abundant in the related groups of fungi. One would naturally expect 
these pycnidia to reappear in lichens, and it might be considered somewhat 
arbitrary to classify pycnidia in Sphaeropsideae as asexual reproductive 
organs, and then to regard the very similar structures in lichens as sexual 
spermogonia. It has also been pointed out that when undoubted pycnidia 
do occur on the lichen thallus, as in Calicium, Strigula, Peltigera, etc., they 
in no way differ from structures regarded as spermogonia except in the size 
of the pycnidiospores and, even among these, there are transition forms. 
The different types of spermatia can be paralleled among the fungal pyc- 
nidiospores and the same is also true as regards the sporophores generally. 
Those described as arthrosterigmata by Nylander as endosporous by 
Steiner were supposed to be peculiar to lichens; but recently Laubert 1 has 
described a fungal pycnidium which grew on the trunk of an apple tree and 
in which the spores are not borne on upright sporophores but are budded 
off from the cells of the plectenchyma lining the pycnidium. It may be that 
future research will discover other such instances, though that type of sporo- 
phore is evidently of very rare occurrence among fungi. 

b. COMPARISON WITH FUNGI. The most obvious spermogonia among 
fungi with which to compare those of lichens occur in the Uredineae where 
they are associated with the life-cycle of a large number of rust species. 
They are small flask-shaped structures very much like the simpler forms of 
pycnidia and they produce innumerable spermatia which are budded off from 
the tips of simple spermatiophores. The mature spermatium has a delicate 
cell-wall and contains a thin layer of cytoplasm with a dense nucleus which 
occupies almost the whole cavity, cytological characters which, as Blackman 2 
has pointed out, are characteristic of male cells and are not found in any 
asexual reproductive spores. If we accept Istvanm's 3 description and figures 
of the lichen spermatia as correct, their structure is wholly different : there 
being a very small nucleus in the centre of the cell comparable in size with 
those of the vegetative hyphae (Fig. 1 15). 

1 Laubert 1911. 2 Blackman 1904. 3 Istvanffi 1895. 



SPERMOGONIA 207 

Lichen " spermatia " also differ very strikingly from the male cells of any 
given group of plants in their very great diversity of form and size; but the 




a 

Fig. 115. a, spermatia; b, hypha produced from spermatium of 
Buellia punctiformis Th. Fr. XQSO (after Istvanffi). 

chief argument adduced by the opponents of the sexual theory is the capacity 
of germination that has been proved to exist in a fair number of species. It 
is true that germination has been induced in the spermatia of the Uredines by 
several research workers by Plowright 1 , Sappin-Trouffy 2 and by Brefeld 3 
who employed artificial nutritive solutions (sugar or honey), but the results 
obtained were not much more than the budding process of yeast cells. Bre- 
feld also succeeded in germinating the " spermatia " of a pyrenomycetous 
fungus, Polystigma rubntm, one of the germinating tubes reaching a length 
four times that of the spore; but it is now known that all of these fungal 
spermatia are non-functional, either sexually or asexually, and degenerate 
soon after their expulsion, or even while still in the spermogonium. 

c. INFLUENCE OF SYMBIOSIS. In any consideration of lichens it is 
constantly necessary to hark back to their origin as symbiotic organisms, 
and to bear in mind the influence of the composite life on their development. 
After germination from the spore, the lichen hypha is so dependant on its 
association with the alga, that, in natural conditions, though it persists 
without the gonidia for a time, it attains to only a rather feeble growth of 
mycelial filaments. In nutritive cultures, as Moller has proved, the absence 
of the alga is partly compensated by the artificial food supply, and a scanty 
thalline growth is formed up to the stage of pycnidial fruits. Not only in 
pycnidia but in all the fruiting bodies of lichens, symbiosis has entailed 
a distinct retrogression in the reproductive importance of the spores, as 
compared with fungi. 

In Ascomycetes, the asci constitute the overwhelming bulk of the 
hymenium ; in most lichens, there are serried ranks of paraphyses with 
comparatively few asci, and the spores are often imperfectly developed. 
It would not therefore be surprising if the bodies claimed by Moller and 
others as pycnidiospores had also lost even to a considerable extent their 
reproductive capacity. 

1 Plowright 1889. 2 Sappin-Trouffy 1896. 3 Brefeld 1891. 



208 REPRODUCTION 

d. VALUE IN DIAGNOSIS. Lichen spermpgonia have once and again 
been found of value in deciding the affinity of related plants, and though 
there are a number of lichens in which we have no record of their occurrence, 
they are so constant in others, that they cannot be ignored in any true 
estimation of species. Nylander laid undue stress on spermogonial characters, 
considering them of almost higher diagnostic value than the much more 
important ascosporous fruit. They are, after all, subsidiary organs, and 
often especially in crustaceous species they are absent, or their relation 
to the species under examination is doubtful. 



CHAPTER V 

PHYSIOLOGY 
I. CELLS AND CELL PRODUCTS 

ANY study of cells or cell- membranes in lichens should naturally include 
those of both symbionts, but the algae though modified have not been 
profoundly changed, and their response to the influences of the symbiotic 
environment has been already described in the discussion of lichen gonidia. 
The description of cells and their contents refers therefore mainly to the 
fungal tissues which form the framework of the plant ; they have been 
transformed by symbiosis to lichenoid hyphae in some respects differing 
from, in others resembling, the fungal hyphae from which they are derived. 

A. CELL-MEMBRANES 

a. CHITIN. It was recognized by workers in the early years of the 
nineteenth century that the substance forming the cell-walls of fungal 
hyphae differed very markedly from the cellulose of the membranes in other 
groups of plants, the blue colouration with iodine and sulphuric acid so 
characteristic of cellulose being absent in most fungi. Various explanations 
were suggested ; but it was always held that the doubtful substance was a 
cellulose containing something peculiar to fungi, this view being strengthened 
by the fact that, after long treatment with potash, a blue reaction was 
obtained. It was called fungus-cellulose by De Bary 1 in order to distinguish 
it from true cellulose. 

It was not till a much later date that any exact work was done on the 
fungal cell, and that Gilson 2 by his researches was able to prove that the 
membranes of fungi contained probably no cellulose, or, "if cellulose were 
present, it was in a different condition from the cellulose of other plants." 
Winterstein 3 followed with the results of his examination of fungus-cellulose: 
he found that it contained nitrogen and therefore differed very considerably 
from typical plant cellulose. Gilson 4 published a second paper dealing 
entirely with fungal tissues in which he also established the presence of 
nitrogen, and added that this nitrogenous compound resembled in various 
ways the chitin 8 of animal cells. He further discovered that by heating it 
with potash a substance was obtained that took a reddish-violet stain when 
treated with* iodine and weak sulphuric acid. This substance, called by him 
mycosin, was proved later to be similar to chitosan 5 , a product of chitin. 

1 De Bary 1866, p. 7. 2 Gilson 1893. 3 Winterstein 1893. 4 Gilson 1894. 

5 The chemical formula of chitin \ given as CaoHiooNgOas, that of chitosan as CuH^NaOio- 

S. L. 14 



210 PHYSIOLOGY 

Escombe 1 analysed the hyphal membranes of Cetraria and found that 
they consisted mainly of a body called by him lichenin and of a para- 
galactan. From Peltigera he extracted a substance with physical properties 
agreeing fairly well with those of chitosan, though analysis did not give 
percentages reconcilable with that substance; the yield however was very 
small. No lichenin was detected. 

Van Wisselingh 2 examined the hyphae of lichens as well as of fungi and 
experimented with a considerable number of both types of plants. He 
succeeded in proving the presence of chitin in the higher fungi (Basidio- 
mycetes and Ascomycetes) and in lichens with one or two exceptions 
. (Cladonia and Cetraria}. Though in some the quantity found was exceed- 
ingly small, in others, such as Peltigera, the walls of the hyphae were 
extremely chitinous. More recently Wester 3 has gone into the question as 
regards lichens, and he has practically confirmed all the results previously 
obtained by Wisselingh. In some species, as for instance in Cladonia rangi- 
ferina, Cl. squamosa, Cl. gracilis, Ramalina calicaris, Solorina crocea and 
others, he found that chitin existed in large quantities, while in Evernia 
prunastri, Usnea florida, U. artiailata, Sticta damaecornis and Parmelia 
saxatilis very little was present. The variation in the amount present may 
be very great even in the species of one genus : none for instance has been 
detected in Cetraria islandica nor in C. nivalis while it is abundant in other 
Cetrariae. There is also considerable variation in quantity in different 
individuals of the same species, and even in different parts of the thallus 
of one lichen. Factors such as habitat, age of the plant, etc., may, however, 
account to a considerable extent for the differences in the results obtained. 

b. LICHENIN AND ALLIED CARBOHYDRATES. It has been proved, as 
already stated, that chitin is present in the hyphal cell-walls of all the lichens 
examined except in those of Cetraria islandica (Iceland Moss), C. nivalis 
and, according to Wester 3 , in those of Bryopogon (Alectoriae). In these 
lichens another substance of purely carbohydrate nature is the chief consti- 
tuent of the cell-walls which swell up when soaked in water to a colourless 
gelatinous substance. 

Berzelius 4 first drew attention to the peculiar qualities of this lichen 
product, and, recognizing its resemblance in many respects to ordinary starch, 
he called it " lichen-starch " or " moss-starch." More exact observations were 
made later by Guerin-Varry 5 who described its properties and showed by 
his experiments that it contained no admixture of either starch or gum. He 
adopted the name lichenin for this organic soluble part of Iceland Moss. 
An analysis of lichenin was made by Mulder 6 who detected in addition to 
lichenin, which coloured yellow with iodine, small quantities of a blue- 

1 Escombe 1896. 2 Wisselingh 1898 3 Wester 1909. 4 Berzelius 1813. 

5 Guerin-Varry 1834. s Mulder 1838. 



CELLS AND CELL PRODUCTS 211 

colouring substance which could be dissolved out from the lichenin and 
which he considered to be true starch. Berg 1 also demonstrated the com- 
pound nature of lichenin: he isolated two isomerous substances with the 
formula C 6 H 10 O 5 . The name " isolichenin " was given to the second blue- 
colouring substance by Beilstein 2 in 1881. 

More recently Escombe 3 has chemically analysed the cell-wall of Cetraria 
islandica: after the elimination of fat, oil, colouring matter and bitter consti- 
tuents he found that there remained the compound lichenin, an anhydride of 
galactose with the formula C 6 H 10 O 5 , which, as stated above, consists of two 
substances lichenin and isolichenin 4 ; the latter is soluble in cold water and 
gives a blue reaction with iodine, lichenin is only soluble in hot water and is 
not coloured blue. Both are derivatives of galactose, a sugar found in a great 
number of organic tissues and substances, among others in gums. 

Lichenin has also been obtained by Lacour 5 from Lecanora esculenta, an 
edible desert lichen supposed to be the manna of the Israelites. Wisselingh 6 
tested the hymenium of thirteen different lichens for lichenin. He found it 
in the walls of the ascus of all those he examined except Graphis. Everniin, 
a constituent of Evernia prunastri, was isolated and described by Stude 7 . 
It is soluble in water and, though considered by Czapek 8 to be identical 
with lichenin, it differs, according to Ulander", in being dextro-rotatory to 
polarized light; lichenin on the contrary is optically inactive. Escombe 3 
also obtained a substance from Evernia which he considered to be comparable 
with chitosan. Usnein which has been extracted 6 from Usnea barbata 
may also be identical with lichenin, but that has not yet been established. 
Ulander 9 examined chemically the cell-walls of a fairly large number of 
lichens. Cetraria islandica, C. aculeata and Usnea barbata, designated as 
the " Cetraria group," contained soluble mucilage-forming substances similar 
to lichenin. A second " Cladonia group " which included Cl. rangiferina 
with the variety alpestris, Stereocaulon paschale and Peltigera aphthosa yielded 
almost none. After the soluble carbohydrates were removed by hot water, 
the insoluble substances were hydrolysed and the "Cetraria group" was found 
to contain abundant d-glucose with small quantities of d-mannose and 
d-galactose; the "Cladonia group," abundant d-mannose and d-galactose with 
but little d-glucose. Hydrolysis was easier and quicker with the former group 
than with the latter. 

Besides these, which rank as hexosans, Ulander found small quantities 
of pentosans and methyl pentosans. All these substances which are such 
important constituents of the hyphal membranes of lichens are classed by 
Ulander as hemicelluloses of the same nature as mannan, galactan and dex- 
tran, or as substances between hemicellulose and the glucoses represented 

1 Berg 1873. - Beilstein ex Errera 1882, p. 16 (note). 3 Escombe 1896. 4 Wiesner 1900. 
5 Lacour 1880. 6 Wisselingh 1898. 7 Stude 1864. 8 Czapek 1905, I. p. 515. 9 Ulander 1905. 

142 



212 PHYSIOLOGY 

by lichenin, everniin, etc. They are doubtless reserve stores of food material, 
and they are chiefly located in the cell-walls of the medullary hyphae which 
are often so thick as almost to obliterate the lumen of the cells. Ulander 
made no test for chitin in his researches. 

Ulander's results have been confirmed by those obtained by K. Miiller 1 . 
In Cladonia rangiferina, Muller found that the cell-membranes of the hyphae 
contained, as hemicelluloses, pentosans in small quantities and galactan, but 
no lichenin and very little chitin. In Evernia prunastri hemicelluloses formed 
the chief constituents of the thallus, and from it he was able to isolate 
galactan soluble in weak hot acid, and everniin soluble in hot water, the 
latter with the formula C 7 Hi 5 O 6 , a result differing from that obtained by 
Stiide 2 who has given it as C 9 H 14 O 7 ; chitin was also present in small 
quantities. In Ramalina fraxinea, the soluble part of the thallus (in hot 
water) differed from everniin and might probably be lichenin. Cetraria 
islandica was also analysed and yielded various hemicelluloses, chiefly 
dextran and galactan, with less pentosan. No chitin has ever been found in 
this lichen. In testing minute quantities of material for chitin, Wisselingh 3 
heated the tissue in potash to i6oC. The potash was then gradually re- 
placed by glycerine and distilled water; the precipitate was placed on a slide 
and the preparation stained under the microscope by potassium-iodide-iodine 
and weak sulphuric acid. Chitin, if present, would have been changed by 
the potash to mycosin which gives a violet colour with the staining solution. 

It has been stated by Schellenberg 4 that these lichen membranes may 
become lignified. He obtained a red reaction with phloroglucine test 
for lignin in Cetraria islandica and Cladonia furcata. Further research is 
required. 

c. CELLULOSE. Several workers claim to have found true cellulose in 
the cell-walls of the hyphal tissues of a few lichens ; but the more careful 
analyses of Escombe 5 Wisselingh 3 and Wester 6 have disproved their results. 
The cell-walls of all the gonidia, however, are formed of cellulose, or according 
to Escombe of glauco-cellulose, except those of Peltigera in which Wester 
found neither cellulose nor chitin. Czapek 7 suggests that the blue reaction 
with iodine characteristic of the cell-walls in some apothecia, of the asci and 
of the hyphae in cortex or medulla in a few instances, may be due to the 
presence of carbohydrates of the nature of galactose. Moreau 8 in a recent 
paper terms the substance that gives a blue reaction with iodine at the tips 
of the asci " amyloid." In Peltigera the ascus tip is occupied by such a plug 
of amyloid which at maturity is projected like a cork from the ascus and 
may be found on the surface of the hymenium. 

1 Muller 1905. 2 Stude 1864. 3 Wisselingh 1898. 4 Schellenberg 1896. 

5 Escombe 1896. 6 Wester 1909. 7 Czapek 1905, I. p. 515. 8 Moreau 1916. 



CELLS AND CELL PRODUCTS 213 

B. CONTENTS AND PRODUCTS OF THE FUNGAL CELLS 

a. CELL-SUBSTANCES. The cells of lichen hyphae contain protoplasm 
and nucleus with glucoses. It is doubtful if starch has been found in fungal 
hyphae ; it is replaced, in some of the tissues at least, by glycogen, a carbo- 
hydrate (C 6 H 10 O s ) very close to, if not identical with, animal glycogen, a 
substance which is soluble in water and colours reddish-brown (wine-red) 
with iodine. Errera 1 first detected its presence in Ascomycetes where it is 
associated with the epiplasm of the cells, more especially of the asci, and he 
considered it to be physiologically homologous with starch. He included 
lichens, as Ascomycetes, in his survey of fungi and quotes, in support of his 
view that lichen hyphae also contain glycogen, a statement made by Schwen- 
dener 2 that "the contents of the ascogenous hyphae of Coenogonium Linkii 
stain a deep-brown with iodine." Errera also instances the red-brown reaction 
with iodine, described by de Bary 3 , as characteristic of the large spores of 
Ochrolechia (Lecanora}pallescens, while the germinating tubes of these spores 
become yellow with iodine like ordinary protoplasm. Glycogen has been, 
so far, found only in the cells of the reproductive system. 

Iodine was found by Gautier 4 in the gonidia of Parmelia and Peltigera, 
i.e. both in bright-green and blue-green algae. The amount was scarcely 
calculable. 

Herissey 5 claims to have established the presence of emulsin in a large 
series of lichens belonging to such widely separated genera as Cladonia, 
Cetraria, Evernia, Peltigera, Perttisaria, Parmelia, Ramalina, and Usnea. It 
is a ferment which acts upon amygdalin, though its presence has been 
proved in plants such as lichens where no amygdalin has been found*. 
Diastase was demonstrated in the cells of Roccella tinctoria, R. Montagnei 
and oiDendrographa leucophaea by Ronceray 7 who states that, in conjunction 
with air and ammonia, it forms orchil, the well-known colouring substance 
of these lichens. Diastatic ferments have also been determined 8 in Usnea 
florida, Physcia parietina, Parmelia perlata and Peltigera canina. 

b. CALCIUM OXALATE. Oxalic acid (C 2 H 2 O 4 ) is an oxidation product 
of alcohol and of most carbohydrates and in combination is a frequent 
constituent of plant cells. Knop 9 held that it was formed in lichens by the 
reduction and splitting of lichen acids, though, as Zopf 10 has pointed out, 
these are generally insoluble. Hamlet and Plowright 11 demonstrated the 
presence of free oxalic acid in many families of fungi including Pezizae and 
Sphaeriae. The acid combines with calcium to form the oxalate (CaC 2 O 4 ), 
which in the crystalline form is very common in lichens. In the higher 

1 Errera 1882. 2 Schwendener 1862, p. 231. 3 De Bary 1866-1867, p. 211. 4 Gautier 1899. 
5 Herissey 1898. 6 Czapek 1905, II. p. 257. ' Ronceray 1904. 8 Zopf in Schenk 1890, p. 448. 

9 Knop 1872. 10 Zopf 1907. u Hamlet and Plowright 1877. 



2i 4 PHYSIOLOGY 

plants the crystals are formed within the cell, but in lichens they are always 
deposited on the outer surface of the hyphal membranes, mainly of the 
medulla and the cortex. 

Calcium oxalate was first detected in lichens by Henri Braconnot 1 , who 
extracted it by treating the powdered thallus of a number of species (Pertu- 
saria communis, Diploschistes scruposus, etc.) with different reagents. The 
quantity present varies greatly in lichens : Zopf 2 found that it was abundant 
in all the species inhabiting limestone, and states that in such plants the 
more purely lichenic acids are relatively scarce. Errera 3 has calculated the 
amount of calcium oxalate in Lecanora esculenta, a desert lime-loving 
lichen, to be about 60 per cent, of the whole substance of the thallus. 
Euler 4 gives for the same lichen even a larger proportion, 66 per cent, of 
the dry weight. In Pertusaria communis, a corticolous species, the oxalate 
occurs as irregular crystalline masses in the medulla (Fig. 116) and has 

been calculated as 47 per cent, of the 
whole substance. Other crustaceous species 
such as Diploschistes scrufiosus, Haema- 
tomma coccineum, H. ventosum, Lecanora 
saxicola, Lecanora tartarea, etc., contain 
large amounts either in the form of octa- 
hedral crystals or as small granules. 

Rodahl' has recently made obser- 
gonidia; c, medulla; rf, crystal of cal- vations as to the presence of the oxalate 

ciuni oxalate. x ca. 100. ., in / i i_ r> / r\c 

in the thallus of the brown Parmehae. Of 

the fourteen species examined by him, eleven contained calcium oxalate as 
octahedral crystals or as small prisms, often piled up in thick irregular 
masses. Usually the crystals were located in the medullary part of the 
thallus, but in two species, Parmelia verruculifera and P. papulosa, they 
were abundant on the surface cells of the upper cortex. 

c. IMPORTANCE OF CALCIUM OXALATE TO THE LICHEN PLANT. It is 
natural to conclude that a substance of frequent occurrence in any group of 
plants is of some biological significance, and suggestions have not been 
lacking as to the value of oxalic acid or of calcium oxalate in the economy 
of the lichen thallus. Oxalic acid is known to be one of the most efficient 
solvents of argillaceous earth and of iron oxides likely to be in the soil. 
These materials are also conveyed to the thallus as air-borne dust, and would 
thus, with the aid of the acid, be easily dissolved and absorbed. As a direct 
proof of this, Knop 6 has stated that lichen-ash always contains argillaceous 
earth. According to Kratzmann 7 , aluminium, a product of clay, is stored 
up in various lichens. He proved the amount in the ash of Umbilicaria 

1 Braconnot 1825. 2 Zopf 1907. 3 Errera 1893. 4 Euler 1908, p. 7. 

6 Rosendahl 1907. 6 Knop 1872. 7 Kratzmann 1913. 




CELLS AND CELL PRODUCTS 215 

pustulata to be 4/46 per cent., in Usnea barbata 179 pe.r cent., in U. longissima 
considerable quantities while in Roccella tinctoria it occurred in great abun- 
dance. It was also abundant in Diploschistes scruposus, 28' 17 per cent.; it 
declined in Variolaria (Pertusaria) dealbata to 777 per cent., in Cladonia 
rangiferina to 176-2-12 per cent, and in Ramalina fraxinea to r8 per cent. 

Calcium oxalate is directly advantageous to the thallus by virtue of the 
capacity of the crystals to reduce or prevent evaporation, as has been 
pointed out by Zukal 1 . A like service afforded by crystals to the leaves of 
the higher plants in desert lands has been described by Kerner 2 . These 
are frequently encrusted with lime crystals which allow the copious night 
dews to soak underneath them to the underlying cells, while during the day 
they impede, if they do not altogether check, evaporation. 

Calcium oxalate crystals are insoluble in acetic acid, soluble in hydro- 
chloric acid without evolution of gas; they deposit gypsum crystals in 
a solution of sulphuric acid. 

C. OIL-CELLS 

a. OIL-CELLS OF ENDOLITHIC LICHENS. Calcicolous immersed lichens 
are able to dissolve the lime of the substratum, and their hyphae penetrate 
more or less deeply into the rock. In some forms the entire thallus may 
thus be immersed, the fruits alone being visible on the surface of the stone. 
In two such species, Verrncaria calciseda and Petractis (Gyalecta) exantJie- 
matica, Steiner 3 detected peculiar sphaeroid or barrel-shaped cells that 
differed from the other hyphal cells of the thallus, not only in their form, 
but in their greenish-coloured contents. Similar cells were found by Zukal 4 
in another immersed (endolithic) lichen, Verrucaria rupestris f. rosea. He 
describes them as roundish organs crowded on the hyphae and filled with a 
greenish shimmering protoplasm. He 5 found the same types of sphaeroid 
and other swollen cells in the immersed thallus of several calcicolous lichens 
and he finally determined the contents as fat in the form of oil. He found 
also that these fat-cells, though very frequent, were not constantly present 
even in the same species. His observations were confirmed by Hulth 6 for 
a number of allied crustaceous lichens that grow not only on limestone but 
on volcanic rocks. In them he found a like variety of fat-cells intercalary or 
torulose cells, terminal sphaeroid cells and hyphae containing scattered oil- 
drops. Bachmann 7 followed with a study of the thallus of purely calcicolous 
lichens. The specialized oil-cells were fairly constant in the species he 
examined, and, as a rule, they were formed either in the tissues immediately 
below, or at some distance from, the gonidial zone. Funfstuck 8 has also 

1 Zukal 1895, p. 1311. - Kerner and Oliver 1894, p. 235. 3 Steiner 1881. 4 Zukal 1884. 
5 Zukal 1886. 6 Hulth 1891. 7 Bachmann 1892. 8 Funfstuck 1895. 



216 



PHYSIOLOGY 



published an account .of various oil-cells in a large series of calcicolous 
lichens (Fig. 117). 

The occurrence of oil- (or fat-) cells is not dependent on the presence of any 

particular alga as the gonidium of 
the lichen. Funfstuck 1 has described 
the immersed thallus of Opegrapha 
saxicola as one of those richest in 
fat-cells. The gonidia belong to the 
a filamentous alga Trentepohlia um- 
brina and form a comparatively 
thin layer about 160/4 thick near 
the upper surface; isolated algal 
branches may grow down to 350/4 
into the rock, while the fungal ele- 
ments descend to 1 1-5 mm., and 
though the very lowest hyphae were 
without oil as were those imme- 
diately beneath the gonidia the 
interlying filaments, he found, were 
crowded with oil-cells. Sphaeroid 
terminal cells were not present. 

Fiinfstiick 1 has re-examined the 
thallus of Petractis exanthematica, 
an almost wholly immersed lichen 
with a gelatinous gonidium, a species 
of Scytonema. The thallus is homoio- 
merous : the alga forms no special 
zone, it intermingles with the hy- 
phae dow r n to the very base of the 
thallus; the hyphae are extremely 
slender and at the base they measure 
only about I/A in width. Oil-cells 
are abundant in the form of inter- 
calary cells about 3-5/4 in thickness. Nearer the surface sphaeroid cells 
are formed on short lateral outgrowths ; they measure 14-16/4 in diameter 
and occur in groups of 15 to 20. The superficial part of the thallus is a 
mere film ; the hyphae composing it are slightly stouter and more thickly 
interwoven. 

Bachmann 2 and Lang 3 have further described the anatomy of endolithic 
thalli especially with reference to oil-cells, and have supplemented the 
researches of previous workers. New methods of cutting the rock in thin 

1 Fiinfstiick 1899. 2 Bachmann 1904' . 3 Lang 1906. 




Fig. 117. Lecidea immersa Ach. A, sphaeroid 
fat-cells from about 8 mm. below the surface 
x 550. B, oil-hyphae in process of emptying : 
a, sphaercid cells containing oil ; b, cells with 
oil-globules x 600 (after Fiinfstiick). 



CELLS AND CELL PRODUCTS 



217 



slices and of dissolving away the lime enabled them to see the tissues in 
their relative positions. In these immersed lichens, as described by them and 
by previous writers, and more especially in calcicolous species, the gonidial 
zone of Protococcaceous algae lies near the surface of the rock, and is 
mingled with delicate, thin-walled hyphae which usually do not contain oil. 
The more deeply immersed layer is formed of a weft of equally thin-walled 
hyphae, some of the cells of which are swollen and filled with fat globules. 
These oil-cells may occur at intervals along the hyphae or they may form 
an almost continuous row. In addition, strands or bundles of hyphae (Fig. 
1 1 8) containing few or many oil globules traverse the tissue, and true 




Fig. r 1 8. Biatorella (Sarcogyne) simplex Br. and Rostr. 
a, sphaeroid oil-cells ; b, strand of oil-hyphae from 
10-15 mm. below the surface, x 585 (after Lang). 

sphaeroid cells are generally present. These latter arise in great numbers 
on short lateral branchlets, usually near the tip of a filament and the groups 
of cells are not unlike bunches of grapes. Sometimes the oil-cells are massed 
together into a complex tissue. Hyphae from this layer pierce still deeper 
into the rock and constitute the rhizoidal portion of the thallus. They also 
produce sphaeroid oil-cells in great abundance (Fig. 119). In the immersed 





Fig. 119. Biatorella pruinosa Mudd. a, complex of sphaeroid 
oil-cells from lomm. below the surface; t>, hypha of sphaeroid 
cells also from inner part of the thallus. x 585 (after Lang). 




218 PHYSIOLOGY 

thallus of Sarcogyne (Biatorella) pruinosa Lang 1 estimated the gonidial zone 
as 1 75-200 /A in thickness, while the colourless hyphae penetrated the rock 
to a depth of quite 15 mm. 

b. OIL-CELLS OF EPILITHIC LICHENS. The general arrangement of the 
tissues and the occurrence and form of the oil-cells vary in the different 
species according to the nature of the substratum. This has been clearly 
demonstrated by Bachmann 2 in Aspicilia (Lecanora} calcarea, an almost 
exclusively calcareous lichen as the name implies. 
On limestone, he found sphaeroid cells formed in 
great abundance on the deeply penetrating rhi- 
zoidal hyphae (Fig. 120). On a non-calcareous 
brick substratum 3 , a specimen had grown which of 
necessity was epilithic. The cortex and gonidial 
Fig. 120. Lecanora (Aspi- zone together were 40 ft thick; immediately below 
cilia) cah-area Sommerf. there were hyphae with irregular cells free from oil ; 

Early stage of sphaeroid 

cell formation x 175 (after lower still there was formed a compact tissue of 
globose fat-cells. In this case the calcareous lichen 

still retained the capacity to form oil-cells on the non-calcareous impene- 
trable substance. 

Very little oil is formed, as a rule, in the cells of siliceous crustaceous 
lichens which are almost wholly epilithic, but Bachmann found a tissue of 
oil-cells in the thallus of Lecanora caesiocinerea, from Labrador, on a granite 
composed of quartz, orthoclase and traces of mica. A thallus of the same 
species collected in the Tyrol, though of a thicker texture, contained no oil. 
Bachmann 3 suggests no explanation of the variation. 

On granite, rhizoidal hyphae penetrate the rock to a slight extent 
between the different crystals, but only in connection with the mica 4 are 
typical sphaeroid cells formed. 

More or less specialized oil-cells have been demonstrated by Fiinfstiick 5 
in several superficial (epilithic) lichens which grow on a calcareous sub- 
stratum, as for instance Lecanora (Placodium] decipiens, Lecanora crassa and 
other similar species. The oil in these lichens is usually restricted to more or 
less swollen or globose cells; but it may also be present in the ordinary 
hyphae as globules. Zukal 6 found that the smooth little round granules 
sprinkled over the thallus of the soil-lichens, Baeomyces roseus and B. nifus, 
contained in the hyphae typical sphaeroid oil-cells and that they were 
specially well developed in specimens from Alpine situations. In still another 
soil-lichen, Lecidea granulosa, shimmering green oil was found in short-celled 
torulose hyphae. 

Rosendahl's 7 researches on the brown Parmeliae resulted in the unex- 

1 Lang 1906, p. 171. 2 Bachmann 1892. 3 Bachmann 1904 1 . 4 Bachmann 1904 *. 

6 Fiinfstuck 1895. Zukal 1895, p. 1372. R OS endahl 1907. 



CELLS AND CELL PRODUCTS 219 

pected discovery of specialized oil-cells situated in the cortices upper and 
lower of five species out of fourteen which he examined. In one of the 
species, P.papulosa, they also occurred in the cortex of the rhizoids. The 
oil-cells were thinner-walled and larger than the neighbouring cortical cells ; 
they were clavate or ovate in form and sometimes formed irregular external 
processes. They were more or less completely filled with oil which coloured 
brown with osmic acid, left a fat stain on paper and, when extracted, burned 
with a shining reddish flame. These oil-cells were never formed in the 
medulla nor in the gonidial region. 

c. SIGNIFICANCE OF OIL-FORMATION. Zukal 1 regarded the oil stored 
in these specialized cells as a reserve product of service to the plant in the 
strain of fruit-formation, or in times of prolonged drought or deprivation of 
light. According to his observations fat was most freely formed in lichens 
when periods of luxuriant growth alternated with periods of starvation. He 
cites, as proof of his view, the frequent presence of empty sphaeroid cells, 
and the varying production of oil affected by the condition, habitat, etc. of 
the plant. Fiinfstiick 2 , on the other hand, considers the oil of the sphaeroid 
and swollen cells as an excretion, representing the waste products of meta- 
bolism in the active tissue, but due chiefly to the presence of an excess of 
carbonic acid which, being set free by the action of the lichen acids on the 
carbonate of lime, forms the basis of fat-formation. He points out that the 
development of fat-cells is always greater in endolithic species in which the 
gonidial layer the assimilating tissue is extremely reduced. In epilithic 
lichens with a wide gonidial zone, the formation of oil is insignificant. He 
states further that if the oil were a direct product of assimilation, the cells 
in which it is stored would be found in contact with the gonidia, and that 
is rarely the case, the maximum of fat production being always at some 
distance. 

Fiinfstuck tested the correctness of his views by a prolonged series of 
growth experiments; he removed the gonidial layer in an endolithic lichen, 
and found that fat storage continued for some time afterwards, its production 
being apparently independent of assimilative activity. The correctness of 
his deductions was further proved by observations on lichens from glacier 
stones. In such unfavourable conditions the gonidia were scanty or absent, 
having died off, but the hyphae persisted and formed oil. He 3 also placed 
in the dark two quick-growing calcicolous lichens, Verrucaria calciseda and 
Opegrapha saxicola. At the end of the experiment, he found that they had 
increased in size without using up the fat. Lang 4 also is inclined to reject 
Zukal's theory, seeing that the fat is formed at a distance from the tissues 
reproductive and others in need of food supply. He agrees with Fiinf- 
stiick that the oil is an excretion and represents a waste-product of the plant. 

1 Zukal 1895. 2 Fiinfstuck 1896. 3 Fiinfstuck 1899. 4 Lang 1906. 



220 PHYSIOLOGY 

Considerable light is thrown on the subject of oil-formation by the results 
of recent researches on the nutrition of algae and fungi. Beijerinck 1 made 
comparative cultures of diatoms taken from the soil, and he found that so 
long as culture conditions were favourable, any fat that might be formed 
was at once assimilated. If, however, some adverse influence checked the 
growth of the organism while carbonic acid assimilation was in full vigour, 
fat was at once accumulated. The adverse influence in this case was the 
lack of nitrogen, and Beijerinck considers it an almost universal rule in plants 
and animals, that where there is absence of nitrogen, in a culture otherwise 
suitable, fat-oils will be massed in those cells which are capable of forming 
oil. He observed that in two of the cultures of diatoms the one which alone 
was supplied with nitrogen grew normally, while the other, deprived of 
nitrogen, formed quantities of oil-drops. Wehmer 2 recordsthe same experience 
in his cultural study of Aspergillus. Sphaeroid fat-cells, similar to those 
described by Zukal in calcicolous lichens, were formed in the hyphae of a 
culture containing an overplus of calcium carbonate, and he judged, entirely 
on morphological grounds, that these were not of the nature of reserve-storage 
cells. 

Stahel 3 has definitely established the same results in cultures of other 
filamentous fungi. In an artificial culture medium in which nitrogen was 
almost wholly absent, the cells of the mycelium seemed to be entirely 
occupied byoil-drops, and this fatty condition he considered to be a symptom 
of degeneration due to the lack of nitrogen. These experiments enable us 
to understand how the hyphae of calcicolous lichens, buried deep in the 
substratum, deprived of nitrogen and overweighted with carbonic acid, may 
suffer from fatty degeneration as shown by the formation of" sphaeroid-cells." 
The connection between cause and effect is more obscure in the case of 
lichens growing on the surface of the soil, such as Baeomyces roseus, or of 
tree lichens such as the brown Parmeliae, but the same influence lack of 
sufficient nitrogenous food may be at work in those as well as in the endo- 
lithic species, though to a less marked extent 

It seems probable that the capacity to form oil- or fat-cells has become 
part of the inherited development of certain lichen species and persists 
through changes of habitat as exemplified in Lecanora calcarea*. 

In considering the question of the formation and the function of fat in 
plant cells, it must be remembered that the service rendered to the life of 
the organism by this substance is a very variable one. In the higher plants 
(in seeds, etc.) fat undoubtedly functions in the same way as starch and 
other carbohydrates as a reserve food. It" is evidently not so in lichens, and 
in one of his early researches, Pfeffer 8 proved that similarly oil was only 

1 Beijerinck 1904. * Wehmer 1891. 3 Stahel 1911. * See p. 218. 

5 Pfeffer 1877. 



CELLS AND CELL PRODUCTS 221 

an excretion in the cells of hepatics. He grew various species in which oil- 
cells occurred in the dark and then tested the cell contents. He found that 
after three months of conditions in which the formation of new carbohydrates 
was excluded, the oil in the cells, instead of having served as reserve material, 
was entirely unchanged and must in that instance be regarded as an 
excretion. 

D. LICHEN-ACIDS 

a. HISTORICAL. The most distinctive and most universal of lichen pro- 
ducts are the so-called lichen-acids, peculiar substances found so far only in 
lichens. They occur in the form of crystals or minute granules deposited in 
greater or less abundance as excretory bodies on the outer surface of the 
hyphal cells. Though usually so minute as scarcely to be recognized as 
crystals, yet in a fairly large series their form can be clearly seen with a 
high magnification. Many of them are colourless; others are a bright yellow, 
orange or red, and give the clear pure tone of colour characteristic of some 
of our most familiar lichens. 

The first definite discovery of a lichen-acid was made towards the begin- 
ning of the nineteenth century and is due to the researches of C. H. Pfaff 1 . 
He was engaged in an examination of Cetraria islandica, the Iceland Moss, 
which in his time was held in high repute, not only as a food but as a tonic. 
He wished to determine the chemical properties of the bitter principle con- 
tained in it, which was so much prized by the Medical Faculty of the period, 
though the bitterness had to be removed to render palatable the nutritious 
substance of the thallus. He succeeded in isolating an acid which he tested 
and compared with other organic acids and found that it was a new substance, 
nearest in chemical properties to succinic acid. In a final note, he states 
that the new :< lichen-acid," as he named it, approached still nearer to boletic 
acid, a constituent of a fungus, though it was distinct from that substance 
also in several particulars. The name " cetrarin " was proposed, at a later 
date, by Herberger 2 who described it as a " subalkaloidal substance, slightly 
soluble in cold water to which it gives a bitter taste; soluble in hot water, 
but, on continued boiling, throwing down a brown powder which is slightly 
soluble in alcohol and readily soluble in ether." Knop and Schnederman 3 
found that Herberger's "cetrarin" was a compound substance and contained 
besides other substances " cetraric acid " and lichesterinic acid. It has now 
been determined by Hesse 4 as fumarprotocetraric acid (C<j2 H^ O^), a deri- 
vative of which is cetraric acid or triaethylprotocetraric acid with the formula 
C 54 H3 9 O 24 (OC 2 H 6 )3 and not C2oHi 8 O 9 as had been supposed. Cetraric acid 
has not yet been isolated with certainty from any lichen 5 . 

1 Pfaff 1826. 2 Herberger 1830. s Knop and Schnederman 1846. 4 Hesse 1904. 
5 Zopf 1907, p. 179. 



222 PHYSIOLOGY 

After this first isolation of a definite chemical substance, further research 
was undertaken, and gradually a number of these peculiar acids were recog- 
nized, the lichens examined being chiefly those that were of real or supposed 
economic value either in medicine or in the arts. In late years a wider 
chemical study of lichen products has been vigorously carried on, and the 
results gained have been recently arranged and published in book form by 
Zopf 1 . Many of the statements on the subject included here are taken from 
that work. Zopf gives a description of all the acids that had been discovered 
up to the date of publication, and the methods employed for extracting each 
substance. The structural formulae, the various affinities, derivatives and 
properties of the acids, with their crystalline form, are set forth along with 
a list of the lichens examined and the acids peculiar to each species. In 
many instances outline figures of the crystals obtained by extraction are 
given. For a fuller treatment of the subject, the student is referred to the 
book itself, as only a general account can be attempted here. 

b. OCCURRENCE AND EXAMINATION OF LICHEN-ACIDS. Acids have 
been found, with few exceptions, in all the lichens examined. They are 
sometimes brightly coloured and are then easily visible under the microscope. 
Generally their presence can only be determined by reagents. Over 140 
different kinds have been recognized and their formulae determined, though 
many are still imperfectly known. As a rule related lichen species contain 
the same acids, though in not a few cases one species may contain several 
different kinds. In growing lichens, they form I to 8 per cent, of the dry 
weight, and as they are practically, while unchanged, insoluble in water, they 
are not liable to be washed out by rain, snow or floods. Their production 
seems to depend largely on the presence of oxygen, as they are always 
found in greatest abundance on the more freely aerated parts of the thallus, 
such as the soredial hyphae, the outer rind or the loose medullary filaments. 
They are also often deposited on the exposed disc of the apothecium, on the 
tips of the paraphyses, and on the wall lining the pycnidia. They are absent 
from the thallus of the Collemaceae, these being extremely gelatinous lichens 
in which there can be little contact of the hyphae with the atmosphere. 
No free acids, so far as is known, are contained in Sticta fuliginosa, but 
a compound substance, trimethylamin, is present in the thallus of that lichen. 
It has also been affirmed that acids do not occur in any Peltigera nor in 
two species of Nephromium, but Zopf 1 has extracted a substance peltigerin 
both from species of Peltigera and from the section Peltidea. 

For purposes of careful examination freshly gathered lichens are most 
serviceable, as the acids alter in herbarium or stored specimens. It is well, 
when possible, to use a fairly large bulk of material, as the acids are often 
present in small quantities. The lichens should be dried at a temperature 

* Zopf .907. 



CELLS AND CELL PRODUCTS 223 

not above 40 C. for fear of changing the character of the contained sub- 
stances, and they should then be finely powdered. When only a small 
quantity of material is available, it has been recommended that reagents 
should be applied and the effect watched under the microscope with a low 
power magnification. This method is also of great service in determining 
the exact position of the acids in the thallus. 

In microchemical examination, Senft 1 deprecates the use of chloroform, 
ether, etc., seeing that their too rapid evaporation leaves either an amorphous 
or crystalline mass of material which does not lend itself to further examina- 
tion. He recommends as more serviceable some oil solution, preferably 
"bone oil" (neat's-foot oil), in which a section of the thallus should be broken 
up under a cover-glass and subjected to a process of slow heating; some 
days must elapse before the extraction is complete. The surplus oil is then 
to be drained off, the section further bruised and the substance examined. 

Acids in bulk should be extracted by ether, acetone, chloroform, 
benzole, petrol-ether and lignoin or by carbon bisulphide. Such solvents as 
alcohols, acetates and alkali solutions should not be used as they tend to 
split up or to alter the constitution of the acids. For the same reason, the 
use of chloroform is to a certain extent undesirable as it contains a percentage 
of alcohol. Ether and acetone, or a mixture of both, are the most efficient 
solvents, and all acids can be extracted by their use, if the material is left 
to soak a sufficient length of time, either in the cold or warmed. It is 
however advisable to follow with a second solvent in case any other acid 
should be present in the tissues. Concentrated sulphuric acid dissolves out 
all acids but often induces colour changes in the process. 

All known lichen-acids form crystals, though the crystalline form may 
alter with the solution used. After filtering and distilling, the residue will 
be found to contain a mixture of these crystals along with other substances, 
which may be removed by washing, etc. 

c. CHARACTER OF ACIDS. Many lichen-acids are more or less bitter to 
the taste; they are usually of an acid nature though certain of the substances 
are neutral, such as zeorin, a constituent of various Lecanoraceae.Physciaceae 
and Cladoniaceae, stictaurin, originally obtained from Sticta aurata, lei- 
phemin, from Haematonima coccineum, and others. 

A large proportion are esters or alkyl salts formed by the union of an 
alcohol and an acid; these are insoluble in alkaline carbonates. It is con- 
sidered probable that the fungus generates the acid, while the alcohol arises 
in the metabolic processes in the alga. It has indeed been proved that the 
alcohol, erythrit, is formed in at least two algae, Protococcus vulgaris and 
Trentepohlia jolithns ; and the lichen-acid, erythrin (CaoH^do), obtained 
from species of Roccella in which the alga is Trentepohlia, is, according to 

1 Senft 1907. 



224 PHYSIOLOGY 

Hesse, the erythrit ester of lecanoric acid (C 16 H 14 O 7 ), a very frequent consti- 
tuent of lichen thalli. It is certain that the interaction of both symbionts is 
necessary for acid production. This was strikingly demonstrated by Tobler 1 
in his cultural study of the lichen thallus. He succeeded in growing, to a 
limited extent, the hyphal part of the thallus of Xanthoria parietina on 
artificial media; but the filaments remained persistently colourless until he 
added green algal cells to the culture. Almost immediately thereafter the 
characteristic yellow colour appeared, proving the presence of parietin, 
formerly known as chrysophanic acid. Tobler's observation may easily be 
verified in plants from natural habitats. A depauperate form of Placodittm 
citrinum consisting mainly of a hypothallus of felted hyphae, with minute 
scattered granules containing algae, was tested with potash, and only the 
hyphae immediately covering the algal granules took the stain; the hypo- 
thallus gave no reaction. 

It has been suggested 2 that when a decrease of albumenoids takes place, 
the quantity of lichen-acid increases, so that the excreted substance should 
be regarded as a sort of waste product of the living plant, "rather than as a 
product of deassimilation." The subject is not yet wholly understood. 

d. CAUSES OF VARIATION IN QUANTITY AND QUALITY OF LICHEN- 
ACIDS. Though it has been proved that lichen-acids are formed freely all 
the year round on any soil or in any region, it happens occasionally that 
they are almost or entirely lacking in growing plants. Schwarz 3 found this 
to be the case in certain plants of Lecanora tartarea, and he suggests that 
the gyrophoric acid contained in the outer cortex of that lichen had been 
broken up by the ammonia of the atmosphere into carbonic acid and orcin 
which is soluble in water, and would thus be washed away by rain. It has 
also been shown by Schwendener 4 and others that the outer layers of the 
older thallus in many lichens slowly perish, first breaking up and then peeling 
off; the denuded areas would therefore have lost, for some time at least, 
their particular acids. Fiinfstuck 5 considers that the difference in the presence 
and amount of acid in the same species of lichen may be due very often 
to variation in the chemical character of the substratum, and this view tallies 
with the results noted by Heber Howe 6 in his study of American Ramali- 
nae. He observed that, though all showed a pale-yellow reaction with potash, 
those growing on mineral substrata gave a more pronouncedly yellow colour. 

M. C. Knowles 7 found that in Ramalina scopulontm the colour reaction 
to potash varied extremely, being more rapid and more intense, the more 
the plants were subject to the influence of the sea-spray. 

Lichen-acids are peculiarly abundant in soredia, and as, in some species, 

1 Tobler 1909. 2 Keegan 1907. 3 Schwarz 1880, p. 264. 4 Schwendener 1863, p. 180. 
8 FiinfstUck 1902. 6 Heber Howe 1913. 7 Knowles 1913. 



CELLS AND CELL PRODUCTS 225 

the thallus forms these outgrowths, or even becomes leprose more freely in 
damp weather, the amount of acids produced may depend on the amount of 
moisture in the atmosphere. 

Their formation is also strongly influenced by light, as is well shown by 
the varying intensity of colour in some yellow thalli. Placodium elegans, 
always a brightly coloured lichen, changes from yellow to sealing-wax red 
in situations exposed to the full blaze of the sun. Haematomma ventosum, 
though greenish-yellow in lowland situations is intensely yellow in the high 
Alps. The same variation of colour is characteristic of Rhizocarpon geo- 
graphicum which is a bright citron-yellow at high altitudes, and becomes 
more greenish in hue as it nears the plains. The familiar foliose lichen 
Xanthoriaparietina is a brilliant orange-yellow in sunny situations, but grey- 
green in the shade, and then yielding only minute quantities of parietin. 
West 1 and others have noted its more luxuriant growth and brighter colour 
when it grows in positions where nitrogenous food is plentiful, such as the 
roofs of farm-buildings, which are supplied with manure-laden dust, and 
boulders by the sea-shore frequented by birds. 

e. DISTRIBUTION OF ACIDS. Some acids, so far as is known, are only to 
be found in one or at most in very few lichens, as for instance cuspidatic 
acid which is present in Ramalina cuspidata, and scopuloric acid, a constituent 
of Ramalina scopulorum, the acids having been held to distinguish by their 
reactions the one plant from the other. 

Others of these peculiar products are abundant and widely distributed. 
Usninic acid, one of the commonest, has been determined in some 70 species 
belonging to widely diverse genera, and atranorin, a substance first discovered 
in Lecanora atra, has been found again many times; Zopf gives a list of 
about 73 species or varieties from which it has been extracted. Another 
widely distributed acid is salazinic acid which has been found by Lettau 2 in 
a very large number of lichens. 

E. CHEMICAL GROUPING OF LICHEN-ACIDS 

Most of these acids have been provisionally arranged by Zopf in groups 
under the two great organic series: I. The Fat series; and II. The Benzole 
or Aromatic series. 

I. LICHEN-ACIDS OF THE FAT SERIES 

Group i. Colourless substances soluble in alkali, the solution not coloured 
by iron chloride. Exs. protolichesterinic acid (C^H^O^ obtained from species 
of Cetraria, and roccellic acid (C^H^O^ from species of Roccella, from 
Lecanora tartarea, etc. 

1 West, W. 1905. 2 I-ettau 1914. 

S. L. I 5 



226 PHYSIOLOGY 

Group 2. Neutral colourless substances insoluble in alkalies, but soluble 
in alcohol, the solution not coloured by iron chloride. Exs. zeorin (C^H^O^, 
a product of widely diverse lichens, such as Lecanora (Zeord) sulphured, 
Haematomma coccineum, Physcia caesia, Cladonia deformis, etc. and barbatin 
(C 9 H 14 O), a product of Usnea barbata. 

Group 3. Brightly coloured acids, yellow, orange or red, all derivatives 
of pulvinic acid (Ci 8 H 12 O 5 ), a laboratory compound which has not been found 
in nature. The group includes among others vulpinic acid (C 19 Hi 4 O 5 ) from 
the brilliant yellow Evernia (Letharia) vulpina, stictaurin (CseH^Og) deposited 
in orange-red crystals on the hyphae of Sticta aurata, and rhizocarpic acid 
(CaeHaoOg) chiefly obtained from Rhizocarpon geographicum : it crystallizes 
out in slender citron-yellow prisms. 

Group 4. Only one acid, usninic (Ci 8 H ]6 O 7 ), a derivative of acetylacetic 
acid, is placed in this group. It is of very wide-spread occurrence, having 
been found in at least 70 species belonging to very different genera and 
families of crustaceous shrubby and leafy lichens. Zopf himself isolated it 
from 48 species. 

Group 5. The thiophaninic acid (d 2 H 6 O 9 ) group representing only a 
small number. They are all sulphur-yellow in colour and soluble in alcohol, 
the solution becoming blackish-green or dirty blue on the addition of iron 
chloride, with one exception, that of subauriferin obtained from the yellow- 
coloured medulla of Parmelia subaurifera which stains faintly wine-red in 
an iron solution. Thiophaninic acid, which gives its name to this group, 
occurs in Pertusaria lutescens and P. Wulfenii, both of which are yellowish 
crustaceous lichens growing mostly on the trunks of trees. 

II. LICHEN-ACIDS OF THE BENZOLE SERIES 

The larger number of lichen-acids belong to this series, of which 94 at 
least are already known. They are divided into two subseries: I. Orcine 
derivatives, and II. Anthracene derivatives. 

SUBSERIES I. ORCINE DERIVATIVES 

Zopf specially insists that the grouping of this series must be regarded 
as only a provisional arrangement of the many lichen-acids that are included 
therein. All of them are split up into orcine and carbonic acid by ammonia 
and other alkalies. On exposure to air, the ammoniacal or alkaline solution 
changes gradually into orceine, the colouring principle and chief constituent 
of commercial orchil. Orcine is not found free in nature. The orcine sub- 
series includes five groups: 

Group i. The substances in this group form, with hypochlorite of lime 
("CaCl"), red-coloured compounds which yield, on splitting, orsellinic acid. 
Zopf enumerates seven acids as belonging to this group, among which is 



CELLS AND CELL PRODUCTS 227 

lecanoric acid (Ci 6 H 14 O 7 ), found in many different lichens, e.g. Roccella tinc- 
toria, Lecanora tartarea, etc.: whenever there is a differentiated pith and 
cortex it occurs in the pith alone. Erythrin (CaoH^Ojo), a constituent of the 
British marine lichen Roccella fuciformi 's, also belongs to this orsellinic group. 
Group 2. Substances which also form red products with CaCl, but do 
not break up into orsellinic acid. Among the most noteworthy are olivetoric 
acid (C 21 H 26 O 7 ), a constituent of Evernia furfur acea, perlatic acid (C^H^Ou,) 
and glabratic acid (C^H^On), which are obtained from species of Parmelia. 

Group 3. Contains three acids of somewhat restricted occurrence. They 
do not form red products with CaCl, and they yield on splitting everninic 
acid. They are: evernic acid (Ci 7 H 16 O 7 ), found in Evernia prunastri var. 
vulgaris, ramalic acid (C 17 H 16 O 7 ) in Ramalina pollinaria, and umbilicaric acid 
(CasH^On) in species of Gyrophora. 

Group 4. The numerous acids of this group are not easily soluble and 
have a very bitter taste. They are not coloured by CaCl ; when extracted 
with concentrated sulphuric acid, the solution obtained is reddish-yellow or 
deep red. Among the most frequent are fumarprotocetraric acid (C^H^Oss), 
the bitter principle of Cetraria islandica, Cladonia rangiferina^ etc., psoromic 
acid (CaoHuOg), obtained from Alectoria implexa, Lecanora varia, Cladonia 
pyxidata and many other lichens, and salazinic acid (Ci 9 H u O 10 ), recorded by 
Zopf as occurring in Stereocaulon salazinum and in several Parmeliae, but 
now found by Lettau 1 to be very wide-spread. He used micro-chemical 
methods and detected its presence in 72 species from twelve different families. 
The distribution of the acid in the thallus varies considerably. 

Group 5. This is called the atranorin group from one of the most im- 
portant members. They are colourless substances and, like the preceding 
group, are not affected by CaCl, but when split they form bodies that colour 
a more or less deep red with that reagent. Atranorin (C 19 Hi 8 O 8 ) is one of 
the most widely spread of all lichen-acids; it occurs in Lecanoraceae, Par- 
meliaceae, Physciaceae and Lecideaceae. Barbatinic acid (C 19 H^O 7 ), another 
member, is found in Usnea ceratina, Alectoria ochroleuca and in a variety of 
Rhizocarpon geographicum. A very large number of acids more or less fully 
studied belong to this group. 

SUBSERIES II. ANTHRACENE DERIVATIVES 

The constituents of this subseries are derived from the carbohydrate 
anthracene, and are characterized by their brilliant colours, yellow, red.brown, 
red-brown or violet-brown. So far, only ten different kinds have been isolated 
and studied. Parietin 2 (Cj 6 H 12 O 5 ), one of the best known, has been extracted 
{wn\Xanthoriaparietina,Placodium murorum and several other bright-yellow 

1 Lettau 1914. 

8 Parietin differs chemically from chrysophanic acid of Rheum, etc. 

15 2 



228 PHYSIOLOGY 

lichens; solorinic acid (C 16 H 14 O 5 ) occurs in orange-red crystals on the hyphae 
of the pith and under surface of Solorina crocea; nephromin (C 16 H 12 O 6 ) is 
found in the yellow medulla of Nephromium lusitanicum ; rhodocladonic acid 
(C 12 H 8 O 6 or C 14 H 10 O 7 ) is the red substance in the apothecia of the red-fruited 
Cladoniae. 

There are, in addition, a short series of coloured substances which are of 
uncertain position. They are imperfectly known and are of rare occurrence. 
An acid containing nitrogen has been extracted from Roccella fuciformis, 
and named picroroccellin 1 (C^HssNgOs). It crystallizes in comparatively large 
prisms, has an exceedingly bitter taste, and is very sparingly soluble. It is 
the only lichen-acid in which nitrogen has been detected. 

One acid at least, belonging to the Fat series, vulpinic acid, which gives the 
greenish-yellow colour to Letharia vulpina, has been prepared synthetically 
by Volkard 2 . 

F. CHEMICAL REAGENTS AS TESTS FOR LICHENS 

The employment of chemical reagents as colour tests in the determination 
of lichen species was recommended by Nylander 3 in a paper published by 
him in 1866. Many acids had already been extracted and examined, and 
as they were proved to be constant in the different species where they 
occurred, he perceived their systematic importance. As an example of the 
new tests, he cited the use of hypochlorite of lime, a solution of which, 
applied directly to the thallus of species of Roccella, produced a bright-red 
"erythrinic" reaction. Caustic potash was also found to be of service in 
demonstrating the presence of parietin in lichens by a beautiful purple 
stain. Many lichenologists eagerly adopted the new method, as a sure and 
ready means of distinguishing doubtful species ; but others have rejected 
the tests as unnecessary and not always to be relied on, seeing that the 
acids are not always produced in sufficient abundance to give the desired 
reaction, and that they tend to alter in time. 

The reagents most commonly in use are caustic potash, generally indi- 
cated by K ; hypochlorite of calcium or bleaching powder by CaCl ; and 
a solution of iodine by I. The sign -f signifies a colour reaction, while 
indicates that no change has followed the application of the test solution. 
Double signs ^ or any similar variation indicate the upper or lower parts of 
the thallus affected by the reagent. In some instances the reaction only 
follows after the employment of two reagents represented thus: K (CaCl) +. 
In such a case the potash breaks up the particular acid and compounds are 
formed which become red, orange, etc., on the subsequent application of 
hypochlorite of lime. 

1 Stenhouse and Groves 1877. 2 Volkard 1894. 3 Nylander 1866. 



CELLS AND CELL PRODUCTS 229 

As an instance of the value of chemical tests, Zopf cites the reaction of 
hypochlorite of lime on the thallus of four different species of Gyrophora, 
the "tripe de roche": 

Gyrophora torrefacta CaCl + . 

polyrhiza CaCl +. 

proboscidea CaCl . 

erosa CaCl I. 

It must however be borne in mind that these species are well differentiated 
and can be recognized, without difficulty, by their morphological characters. 
Experienced systematists like Weddell refuse to accept the tests unless 
they are supported by true morphological distinctions, as the reactions are 
not sufficiently constant. 

G. CHEMICAL REACTIONS IN NATURE 

Similar colour changes may often be observed in nature. The acids of 
the exposed thallus cortex are not unfrequently split up by the gradual 
action of the ammonia in the atmosphere, one of the compounds thus set 
free being at the same time coloured by the alkali. Thus salazinic acid, a 
constituent of several of our native Parmeliae, is broken up into carbonic 
acid and salazininic acid, the latter taking a red colour. Fumarprotocetraric 
acid is acted on somewhat similarly, and the red colour may be seen in 
Cetraria at the base of the thallus where contact with soil containing 
ammonia has affected the outer cortex of the plant. The same results are 
produced still more effectively when the lichen comes into contact with 
animal excrement. 

Gummy exudations from trees which are more or less ammoniacal may 
also act on the thallus and form red-coloured products on contact with the 
acids present. Lecanora (Aspicilta) cinerea is so easily affected by alkalies 
that a thin section left exposed may become red in time owing to the 
ammonia in the atmosphere. 

II. GENERAL NUTRITION 

A. ABSORPTION OF WATER 

Lichens are capable of enduring almost complete desiccation, but though 
they can exist with little injury through long periods of drought, water is 
essential to active metabolism. They possess no special organs for water 
conduction, but absorb moisture over their whole surface. Several inter- 
dependent factors must therefore be taken into account in considering the 
question of absorption : the type of thallus, whether gelatinous or non- 
gelatinous, crustaceous,foliose or fruticose,as also the nature of the substratum 
and the prevailing condition of the atmosphere. 



2 3 o t PHYSIOLOGY 

a. GELATINOUS LICHENS. The algal constituent of these lichens is 
some member of the Myxophyceae and is provided with thick gelatinous 
walls which have great power of imbibition and swell up enormously in 
damp surroundings, becoming reservoirs of water. Species of Collema, for 
instance, when thoroughly wet, weigh thirty-five times more than when 
dry 1 . There are no interstices in the thallus and frequently no cortex in 
these lichens, but the gelatinous substance itself forms on drying an outer 
skin that checks evaporation so that water is retained within the thallus 
for a longer period than in non-gelatinous forms. They probably always 
retain some amount of moisture, as they share with gelatinous algae the 
power of revival after long desiccation. 

Gelatinous lichens are entirely dependent on a surface supply of water: 
their hyphae or rhizinae when present rarely penetrate the substratum. 

b. CRUSTACEOUS NON-GELATINOUS LICHENS. The lichens with this 
type of thallus are in intimate contact with the substratum whether it be 
soil, rock, tree or dead wood. The hyphae on the under surface of the 
thallus function primarily as hold-fasts, but if water be retained in the 
substratum, the lichen will undoubtedly benefit, and water, to some extent, 
will be absorbed by the walls of the hyphae or will be drawn up by capillary 
attraction. In any case, it could only be surface water that would be avail- 
able, as lichens have no means of tapping any deeper sources of supply. 

Lichens are, however, largely independent of the substratum for their 
supply of water. Sievers 2 , who gave attention to the subject, found that 
though some few crustaceous lichens took up water from below, most of 
them absorbed the necessary moisture on the surface or at the edges of the 
thallus or areolae, where the tissue is looser and more permeable. The 
swollen gelatinous walls of the hyphae forming the upper layers of such 
lichens are admirably adapted for the reception and storage -of water, 
though, according to Zukal 3 , less hygroscopic generally than in the larger 
forms. Beckmann 4 proved this power of absorption, possessed by the upper 
cortex, by placing a crustaceous lichen, Haematomma sp., in a damp 
chamber: he found after a while that water had been taken up by the cortex 
and by the gonidial zone, while the lower medullary hyphae had remained dry. 

Herre 5 has recorded an astonishing abundance of lichens from the desert 
of Reno, Nevada, and these are mostly crustaceous forms, belonging to 
a limited number of species. The yearly rainfall of the region is only about 
eight or ten inches, and occurs during the winter months, chiefly as snow. 
It is during that period that active vegetation goes on; but the plants still 
manage to exist during the long arid summer, when their only possible 
water supply is that obtained from the moisture of the atmosphere during 
the night, or from the surface deposit of dews. 

1 Jumelle 1892. 2 Sievers 1908. 3 Zukal 1895. 4 Beckmann 1907. 5 Herre 1911-. 



GENERAL NUTRITION 231 

c. FOLIOSE LICHENS. Though many of the leafy lichens are provided 
with a tomentum of single hyphae, or with rhizinae on the under surface, 
the principal function of these structures is that of attaching the thallus. 
Sievers 1 tested the areas of absorption by placing pieces of the thallus of 
Parmeliae, of Evernia furfuracea, and of Cetraria glauca in a staining 
solution. After washing and cutting sections, it was seen that the coloured 
fluid had penetrated by the upper surface and by the edge of the thallus, 
as in crustaceous forms, but not through the lower cortex. 

By the same methods of testing, he proved that water penetrates not 
only by capillarity between the closely packed hyphae, but also within the 
cells. A considerable number of lichens were used for experiment, and 
great variations were found to exist in the way in which water was taken 
up. It has been proved that in some species of Gyrophora water is absorbed 
from below: in those in which rhizinae are abundant, water is held by them 
and so gradually drawn up into the thallus; the upper cortex in this genus 
is very thick and checks transpiration. Certain other northern lichens such 
as Cetraria islandica, Cladonia rangiferin'a, etc., imbibe water very slowly, 
and they, as well as Gyrophora, are able to endure prolonged wet periods. 

That foliose lichens do not normally contain much water was proved by 
Jumelle 2 who compared the weight of seven different species when freshly 
gathered, and after being dried ; he found that the proportion of fresh weight 
to dry weight showed least variation in Parmelia acetabulum, as ri4 to i ; 
in Xanthoria parietina it was as i'2i to I. 

d. FRUTICOSE LICHENS. There is no water-conducting tissue in the 
elongate thallus of the shrubby or filamentous lichens, as can easily be tested 
by placing the base in water: it will then be seen that the submerged parts 
alone are affected. Many lichens are hygroscopic and become water-logged 
when placed simply in damp surroundings. The thallus of Usnea, for 
instance, can absorb many times its weight of water: a mass of Usnea 
filaments that weighed 3'8 grms. when dry increased to 13-3 grms. after 
having been soaked in water for twelve hours. Schrenk 3 , who made the 
experiment, records in a second instance an increase in weight from 
3-97 grms. to in 8 grms. The Cladoniae retain large quantities of water in 
their upright hollow podetia. The Australian species, Cladonia retepora, the 
podetium of which is a regular network of holes, competes with the Sphagnum 
moss in its capacity to take up water. 

To conclude : as a rule, heteromerous, non-gelatinous lichens do not 
contain large quantities of water, the weight of fresh plants being generally 
about three times only that of the dry weight. Their ordinary water content 
is indeed smaller than that of most other plants, though it varies at once 
with a change in external conditions. It is noteworthy that a number of 

1 Sievers 1908- * Jumelle 1892. 3 Schrenk 1898. 



232 PHYSIOLOGY 

lichens have their habitat on the sea-shore, constantly subject to spray from 
the waves, but scarcely any can exist within the spray of a waterfall, 
possibly because the latter is never-ceasing. 

B. STORAGE OF WATER 

The gonidial algae Gloeocapsa, Scytonema, Nostoc, etc. among Myxophy- 
ceae, Palmella and occasionally Trentepohlia among Chlorophyceae, have 
more or less gelatinous walls which act as a natural reservoir of water for 
the lichens with which they are associated. In these lichens the hyphae 
for the most part have thin walls, and the plectenchyma when formed as 
below the apothecium in' Collema granuliferum, or as a cortical layer in 
Leptogium is a thin-walled tissue. In lichens where, on the contrary, 
the alga is non-gelatinous as generally in Chlorophyceae or where the 
gelatinous sheath is not formed as in the altered Nostoc of the Peltigera 
thallus, the fungal hyphae have swollen gelatinous walls both in the pith 
and the cortex, and not only imbibe but store up water. 

Bonnier 1 had his attention directed to this thickening of the cell-walls 
as he followed the development of the lichen thallus. He made cultures 
from the ascospore of Physcia (Xanthoria) parietina and obtained a 
fair amount of hyphal tissue, the cell-walls of which became thickened, 
but more slowly and to a much less extent than when associated with the 
gonidia. 

He noted also that when his cultures were kept in a continuously moist 
atmosphere there was much less thickening, scarcely more than in fungi 
ordinarily; it was only when they were grown under drier conditions with 
necessity for storage, that any considerable swelling of the walls took place. 
Further he found that the thallus of forms cultivated in an abundance of 
moisture could not resist desiccation as could those with the thicker 
membranes. These latter survived drying up and resumed activity when 
moisture was supplied. 

C. SUPPLY OF INORGANIC FOOD 

As in the higher plants, mineral substances can only be taken up when 
they are in a state of solution. Lichens are therefore dependent on the sub- 
stances that are contained in the water of absorption : they must receive their 
inorganic nutriment by the same channels that water is conveyed to them. 

a. FoLIOSE AND FRUTICOSE LICHENS. These larger lichens are provided 
with rhizinae or with hold-fasts, which are only absorptive to a very limited 
extent ; the main source of water supply is from the atmosphere and the 
salts required in the metabolism of the cell must be obtained there also 

1 Bonnier 1889*. 



GENERAL NUTRITION 233 

from atmospheric dust dissolved in rain, or from wind -borne particles de- 
posited on the surface of the thallus which may be gradually dissolved and 
absorbed by the cortical and growing hyphae. That substances received 
from the atmospheric environment may be all important is shown by the 
exclusive habitat of some marine lichens; the Roccellae, Lichinae, some 
species of Ramalina and others which grow only on rocky shores are almost 
as dependent on sea-water as are the submerged algae. Other lichens, such 
as Hydrothyria venosa and Lecanora lacustris, grow in streams, or on boulders 
that are subject to constant inundation, and they obtain their inorganic food 
mainly, if not entirely, from an aqueous medium. 

Though lichens cannot live in an atmosphere polluted by smoke, they 
thrive on trees and walls by the road-side where they are liable to be almost 
smothered by soil-dust. West 1 has observed that they flourish in valleys 
that are swept by moisture laden winds more especially if near to a high- 
way, where animal excreta are mingled with the dust. The favourite habitats 
of Xanthoria parietina are the walls and roofs of farm-buildings where the 
dust must contain a large percentage of nitrogenous material ; or stones by 
the sea-shore that are the haunts of sea-birds. Sandstede 2 found on the 
island of Riigen that while the perpendicular faces of the cliffs were quite 
bare, the tops bore a plentiful crop of Lecanora saxicola, Xanthoria lychnea 
and Candellariella mtellina. He attributed their selection of habitat to the 
presence of the excreta of sea-birds. As already stated the connection of 
foliose and fruticose lichens with the substratum is mainly mechanical but 
occasionally a kind of semiparasitism may arise. Friedrich 3 gives an instance 
in a species of Usnea of unusually vigorous development. It grew on bark 
and the strands of hyphae, branching from the root-base of the lichen, 
had reached down to the living tissue of the tree-trunk and had penetrated 
between the cells by dissolving the middle lamella. It was possible to find 
holes pierced in the cell-walls of the host, but it was difficult to decide if 
the hyphae had attacked living cells or were merely preying on dead material. 
Lindau 4 held very strongly that lichen hyphae were non-parasitic, and merely 
split apart the tissues already dead, and the instance recorded by Friedrich 
is of rare occurrence 5 . 

That the substratum does have some indirect influence on these larger 
lichens has been proved once and again. Uloth 6 , a chemist as well as a 
botanist, made analyses of plants of Evernia prunastri taken from birch bark 
and from sandstone. Qualitatively the composition of the lichen substances 
was the same, but the quantities varied considerably. Zopf 7 has, more 
recently, compared the acid content of a form of Evernia furfuracea on rock 
with that of the same species growing on the bark of a tree. In the case of 

1 West 1905. 2 Sandstede 1904. 3 Friedrich 1906. 4 Lindau 1895*. 

6 See p. 109. 6 Uloth 1861. " Zopf 1903. 



234 PHYSIOLOGY 

the latter, the thallus produced 4 per cent, of physodic acid and 2'2 per cent, 
of atranorin. In the rock specimen, which, he adds, was a more graceful plant 
than the other, the quantities were 6 per cent, of physodic acid, and 275 per 
cent, of atranorin. In both cases there was a slight formation of furfuracinnic 
acid. He found also that specimens of Evernia prunastri on dead wood 
contained 8*4 per cent, of lichen-acids, while in those from living trees there 
was only 4^4 per cent, or even less. Other conditions, however, might have 
contributed to this result, as Zopf 1 found later that this lichen when very 
sorediate yielded an increased supply of atranoric acid. 

Ohlert 2 , who made a study of lichens in relation to their habitat, found 
that though a certain number grew more or less freely on either tree, rock 
or soil, none of them was entirely unaffected. Usnea barbata, Evernia pru- 
nastri and Parmelia physodes were the most indifferent to habitat; normally 
they are corticolous species, but Usnea on soil formed more slender filaments, 
and Evernia on the same substratum showed a tendency to horizontal growth, 
and became attached at various points instead of by the usual single base. 

b. CRUSTACEOUS LICHENS. The crustaceous forms on rocks are in a 
more favourable position for obtaining inorganic salts, the lower medullary 
hyphae being in direct contact with mineral substances and able to act 
directly on them. Many species are largely or even exclusively calcicolous, 
and there must be something in the lime that is especially conducive to 
their growth. The hyphae have been traced into the limestone to a depth 
of 15 mm. s and small depressions are frequently scooped out of the rock by 
the action of the lichen, thus giving a lodgement to the foveolate fruit. 

On rocks mainly composed of silica, the lichen has a much harder sub- 
stance to deal with, and one less easily affected by acids, though even silica 
may be dissolved in time. Uloth 4 concluded from his observations that the 
relation of plants to the substratum was chemical even more than physical, 
so far as crustaceous species were concerned. He found that the surface of 
the area of rock inhabited was distinctly marked : even such a hard substance 
as chalcedony was corroded by a very luxuriant lichen flora, the border of 
growth being quite clearly outlined. The corrosive action is due he con- 
sidered to the carbon dioxide liberated by the plant, though oxalic acid, so 
frequent a constituent of lichens, may also share in the corrosion. Egeling 5 
made similar observations in regard to the effect of lichen growth on granite 
rocks; and he further noticed that pieces of glass, over which lichens had 
spread, had become clouded, the dulness of the surface being due to a multi- 
tude of small cracks eaten out by the hyphae. Buchet 6 also gives an instance 
of glass which had been corroded by the action of lichen hyphae. It formed 

1 Zopf 1907. 2 Ohlert 1871. 3 See p. 75. 4 Uloth 1861. 

5 Egeling 1881. Buchet 1890. 



GENERAL NUTRITION 235 

part of an old stained window in a chapel that was obscured by a lichen 
growth which adhered tenaciously. When the window was taken down and 
cleaned, it was found that the surface of the glass was covered with small, 
more or less hemispherical pits which were often confluent. The different 
colours in the picture were unequally attacked, some of the figures or draperies 
being covered with the minute excavations, while other parts were intact. 
It happened also, occasionally, that a colour while slightly corroded in one 
pane would be uninjured in another, but the suggestion is made that there 
might in that case have been a difference in the length of attack by the 
lichen. The selection of colours by the lichens might also be influenced by 
some chemical or physical characters. 

Bachmann 1 found that on granite there is equally a selection of material 
by the hyphae: as a rule they avoid the acid silica constituents; while they 
penetrate and traverse the grains of mica which are dissolved by them 
exactly as are lime granules. 

On another rock consisting mainly of muscovite and quartz he 2 found 
that crystals of garnet embedded in the rock were reduced to a powder by 
the action of the lichen. He concludes that the destroying action of the 
hyphae is accelerated by the presence of carbon dioxide given off by the 
lichen, and dissolved in the surrounding moisture. Lang 3 and Stahlecker 4 
have both come to the conclusion that even the quartz grains are corroded 
by the lichen hyphae. Stahlecker finds that they change the quartz into 
amorphous silicic acid, and thus bring it into the cycle of organic life. Chalk 
and magnesia are extracted from the silicates where no other plant could 
procure them. Lichens are generally rare on pure quartz rocks, chiefly, 
however, for the mechanical reason that the structure is of too close a grain 
to afford a foothold. 

D. SUPPLY OF ORGANIC FOOD 

a. FROM THE SUBSTRATUM. The Ascomycetous fungi, from which so 
many of the lichens are descended, are mainly saprophytes, obtaining their 
carbohydrates from dead plant material, and lichen hyphae have in some 
instances undoubtedly retained their saprophytic capacity. It has been 
proved that lichen hyphae, which naturally could not exist without the 
algal symbiont, may be artificially cultivated on nutrient media without the 
presence of gonidia, though the chief and often the only source ot carbon 
supply is normally through the alga with which the hyphae are associated 
in symbiotic union. 

A large number of crustaceous lichens grow on the bark of trees, and 
their hyphae burrow among the dead cells of the outer bark using up the 

1 Bachmann 1904. 2 Bachmann 1911. 3 Lang 1903. 4 Stahlecker 1906. 



23 6 PHYSIOLOGY 

material with which they come in contact Others live on dead wood, palings, 
etc. where the supply of disintegrated organic substance is even greater ; or 
they spread over withered mosses and soil rich in humus. 

b. FROM OTHER LICHENS. Bitter 1 has recorded several instances ob- 
served by him of lichens growing over other lichens and using up their 
substance as food material. Some lichens are naturally more vigorous than 
others, and the weaker -or more slow growing succumb when an encounter 
takes place. Pertusaria globulifera is one of these marauding species; its 
habitat is among mosses on the bark of trees, and, being a quick grower, it 
easily overspreads its more sluggish neighbours. It can scarcely be considered 
a parasite, as the thallus of the victim is first killed, probably by the action 
of an enzyme. 

Lecanora subfusca and allied species which have a thin thallus are 
frequently overgrown by this Pertusaria and a dark line generally precedes 
the invading lichen; the hyphae and the gonidia of the Lecanorae are first 
killed and changed to a brown structureless mass which is then split up by 
the advancing hyphae of the Pertusaria into small portions. A little way 
back from the edge of the predatory thallus the dead particles are no longer 
visible, having been dissolved and completely used up. Pertusaria amara 
also may overgrow Lecanorae, though, generally, its onward course is 
checked and deflected towards a lateral direction; if however it is in a young 
and vigorous condition, it attacks the thallus in its path, and ahead of it 
appears the rather broad blackish line marking the fatal effect of the enzyme, 
the rest of the host thallus being unaffected. Neither Pertusaria seems to 
profit much, and does not grow either faster or thicker; the thallus appears 
indeed to be hindered rather than helped by the encounter. Biatora (Lecidea) 
quernea with a looser, more furfuraceous thallus is also killed and dissolved 
by Pertusariae; but if the Biatora is growing near to a withering or dead 
lichen it, also, profits by the food material at hand, grows over it and uses it up. 
Bitter has also observed lichens overgrown by Haematomma sp. ; the growth 
of that lichen is indeed so rapid that few others can withstand its approach. 

Another common rock species, Lecanora sordida (L. glaucoma), has a 
vigorous thallus that easily ousts its neighbours. Rhizocarpon geographicu m, 
a slow-growing species, is especially liable to be attacked ; from the thallus 
of L. sordida the hyphae in strands push directly into the other lichen in a 
horizontal direction and split up the tissues, the algae persist unharmed for 
some time, but eventually they succumb and are used up; the apothecia, 
though more resistant than the thallus, are also gradually undermined and 
hoisted up by the new growth, till finally no trace of the original lichen is 
left. Lecanora sordida is however in turn invaded by Lecidea insularis 
(L. intumescens} which is found forming small orbicular areas on the 

1 Bitter 1899. 



GENERAL NUTRITION 237 

Lecanora thallus. It kills its host in patches and the dead material mostly 
drifts away. On any strands that are left Candellariella vitellina generally 
settles and evidently profits by the dead nutriment. It does not spread to 
the living thallus. Lecanora polytropa also forms colonies on these vacant 
patches, with advantage to its growth. 

Even the larger lichens are attacked by these quick-growing crusts. 
Pertitsaria globulifera spreads over Parmelia perlata and P. physodes, 
gradually dissolving and consuming the different thalline layers; the lower 
cortex of the victim holds out longest and can be seen as an undigested 
black substance within the Pertitsaria thallus for some time. As a rule, 
however, the lichens with large lobes grow over the smaller thalli in a purely 
mechanical fashion. 

c. FROM OTHER VEGETATION. Zukal 1 has given instances of association 
between mosses and lichens in which the latter seemed to play the part of 
parasite. The terricolous species Baeomyces rufus (Sphyridium) and Biatora 
decolorans, as well as forms of Lepraria and Variolarta, he found growing 
over mosses and killing them. Stems and leaves of the moss Plagiothecium 
sylvaticum were grown through and through by the hyphae of a Pertusaria, 
and he observed a leaf of Polytrichum commune pierced by the rhizinae of 
a minute Cladonia squamule. The cells had been invaded and the neigh- 
bouring tissue was brown and dead. 

Perhaps the most voracious consumer of organic remains is Lecanora 
tartarea, more especially the northern form frigida. It is the well-known 
cud-bear lichen of West Scotland, and is normally a rock species. It has 
an extremely vigorous thickly crustaceous and quick-growing thallus, and 
spreads over everything that lies in its path decaying mosses, dead leaves, 
other lichens, etc. Kihlman 2 has furnished a graphic description of the way 
it covers up the vegetation on the high altitudes of Russian Lapland. More 
than any other plant it is able to withstand the effect of the cold winds that 
sweep across these inhospitable plains. Other plant groups at certain seasons 
or in certain stages of growth are weakened or killed by the extreme cold 
of the wind, and, immediately, a growth of the more hardy grey crust of 
Lecanora tar tar ea begins to spread over and take possession of the area 
affected very frequently a bank of mosses, of which the tips have been 
destroyed, is thus covered up. In the same way the moorland Cladoniae, 
C. rangiferina (the reindeer moss) and some allied species, are attacked. 
They have no continuous cortex, the outer covering of the long branching 
podetia being a loose felt of hyphae; they are thus sensitive to cold and 
liable to be destroyed by a high wind, and their stems, which are blackened 
as decay advances, become very soon dotted with the whitish-grey crust of 
the more vigorous and resistant Lecanora. 

1 Zukal 1879. 2 Kihlman 1890. 



238 PHYSIOLOGY 

III. ASSIMILATION AND RESPIRATION 
A. INFLUENCE OF TEMPERATURE 

a. HIGH TEMPERATURE. It has been proved that plants without chloro- 
phyll are less affected by great heat than those that contain chlorophyll. 
Lichens in which both types are present are more capable of enduring high 
temperatures than the higher plants, but with undue heat the alga succumbs 
first. In consequence, respiration, by the fungus alone, can go on after 
assimilation (photosynthesis) and respiration in the alga have ceased. 

Most Phanerogams cease assimilation and respiration after being sub- 
jected for ten minutes to a temperature of 50 C. Jumelle 1 made a series of 
experiments with lichens, chiefly of the larger fruticose or foliaceous types, 
with species ofRamatitia, Physcia and Parmelia, also with Evernia prunastri 
and Cladonia rangiferina. He found that as regards respiration, plants 
which had been kept for three days at 45 C., fifteen hours at 50, then five 
hours at 60, showed an intensity of respiration almost equal to untreated 
specimens, gaseous interchange being manifested by an absorption of oxygen 
and a giving up of carbon dioxide. 

The power of assimilation was more quickly destroyed : as a rule it 
failed after the plants had been subjected successively to a temperature of 
one day at 45 C., then three hours at 50 and half-an-hour at 60. The 
assimilating green alga, being less able to resist extreme heat, as already 
stated, succumbed more quickly than the fungus. Jumelle also gives the 
record of an experiment with a crustaceous lichen, Lecidea (Lecanora) sul- 
phurea, a rock species. It was kept in a chamber heated to 50 for three 
hours and when subsequently placed in the sunlight respiration took place 
but no assimilation. 

Very high temperatures may be endured by lichen plants in quite natural 
conditions, when the rock or stone on which they grow becomes heated by 
the sun. Zopf 2 tested the thalli of crustaceous lichens in a hot June, under 
direct sunlight, and found that the thermometer registered 55C. 

b. Low TEMPERATURE. Lichens support extreme cold even better than 
extreme heat. In both cases it is the power of drying up and entering at 
any season into a condition of lowered or latent vitality that enables them 
to do so. In winter during a spell of severe cold they are generally in a 
state of desiccation, though that is not always the case, and resistance to 
cold is not due to their dry condition. The water of imbibition is stored in 
the cell-walls and it has been found that lichens when thus charged with 
moisture are able to resist low temperatures, even down to 40 C. or - 50 
as well as when they are dry. Respiration in that case was proved by 

1 Jumelle 1892. - Zopf 1890, p. 489. 



ASSIMILATION AND RESPIRATION 239 

Jumelle 1 to continue to 10, but assimilation was still possible at a tem- 
perature of 40 : Evernia prunastri exposed to that extreme degree of cold, 
but in the presence of light, decomposed carbon dioxide and gave off 
oxygen. 

B. INFLUENCE OF MOISTURE 

a. ON VITAL FUNCTIONS. Gaseous interchange has been found to vary 
according to the degree of humidity present 1 . In lichens growing in sheltered 
positions, or on soil, there is less complete desiccation, and assimilation and 
respiration may be only enfeebled. Lichens more exposed to the air those 
growing on trees, etc. dry almost completely and gaseous interchange may 
be no longer appreciable. In severe cold any water present would become 
frozen and the same effect of desiccation would be produced. At normal 
temperatures, on the addition of even a small amount of moisture the 
respiratory and assimilative functions at once become active, and to an in- 
creasing degree as the plant is further supplied with water until a certain 
optimum is reached, after which the vital processes begin somewhat to 
diminish. 

Though able to exist with very" little moisture, lichens do not endure 
desiccation indefinitely, and both assimilation and respiration probably cease 
entirely during very dry seasons. A specimen of Cladonia rangiferina was 
kept dry for three months, and then moistened: respiration followed but it 
was very feeble and assimilation had almost entirely ceased. Somewhat 
similar results were obtained with Ramalina farinacea and Usiiea barbata. 

In normal conditions of moisture, and with normal illumination, assimi- 
lation in lichens predominates over respiration, more carbon dioxide being 
decomposed than is given forth; and Jumelle has argued from that fact, 
that the alga is well able to secure from the atmosphere all the carbon 
required for the nutrition of the whole plant. The intensity of assimilation, 
however, varies enormously in different lichens and is generally more powerful 
in the larger forms than in the crustaceous : the latter have often an extremely 
scanty thallus and they are also more in contact with the substratum rock, 
humus or wood on which they may be partly saprophytic, thus obtaining 
carbohydrates already formed, and demanding less from the alga. 

An interesting comparison might be made with fungi in regard to which 
many records have been taken as to their possible duration in a dry state, 
more especially on the viability of spores, i.e. their persistent capacity of 
germination. A striking instance is reported by Weir 2 of the regeneration of the 
sporophores of Polystictus sanguineus, a common fungus of warm countries. 
The plant was collected in Brazil and sent to Munich. After about two years 
in the mycological collection of the University, the branch on which it grew 

1 Jumelle 1892. 2 Weir 1919. 



2 4 o PHYSIOLOGY 

was exposed in the open among other branches in a wood while snow still 
lay on the ground. In a short time the fungus revived and before the end 
of spring not only had produced a new hymenium, but enlarged its hymenial 
surface to about one-fourth of its original size and had also formed one 
entirely new, though small, sporophore. 

b. ON GENERAL DEVELOPMENT. Lichens are very strongly influenced 
by abundance or by lack of moisture. The contour of the large majority of 
species is concentric, but they become excentric owing to a more vigorous 
development towards the side of damper exposure, hence the frequent one- 
sided increase of monophyllous species such as Umbilicariapustulata. Wainio 1 
observed that species of Cladonia growing in dry places, and exposed to full 
sunlight, showed a tendency not to develop scyphi, the dry conditions 
hindering the full formation of the secondary thallus. As an instance may 
be cited Cl.foliacea, in which the primary thallus is much the most abundantly 
developed, its favourite habitat being the exposed sandy soil of sea-dunes. 

Too great moisture is however harmful: Nienburg 2 has recorded his 
observations on Sphyridium (Baeomyces rufus): on clay soil the thallus was 
pulverulent, while on stones or other dryer substratum it was granular 
warted or even somewhat squamulose. 

Parmeliaphysodes rarely forms fruits, but when growing in an atmosphere 
constantly charged with moisture 8 , apothecia are more readily developed, 
and the same observation has been made in connection with other usually 
barren lichens. It has been suggested that, in these lichens, the abrupt change 
from moist to dry conditions may have a harmful effect on the developing 
ascogonium. 

The perithecia of Pyrenula nitida are smaller on smooth bark 4 such as 
that of CoryluS) Carpinus, etc., probably because the even surface does not 
retain water. 

IV. ILLUMINATION OF LICHENS 
A. EFFECT OF LIGHT ON THE THALLUS 

As fungi possess no chlorophyll, their vegetative body has little or no 
use for light and often develops in partial or total darkness. In lichens the 
alga requires more or less direct illumination; the lichen fungus, therefore, 
in response to that requirement has come out into the open : it is an adapta- 
tion to the symbiotic life, though some lichens, such as those immersed 
in the substratum, grow with very little light. Like other plants they are 
sensitive to changes of illumination: some species are shade plants, while 
others are as truly sun plants, and others again are able to adapt themselves 
to varying degrees of light. 

1 Wainio 1897, p. 16. 2 Nienburg 1908. 3 Metzger 1903. * Bitter 1899. 



ILLUMINATION OF LICHENS 241 

Wiesner 1 made a series of exact observations on what he has termed 
the " light-use " of various plants. He took as his standard of unity for the 
higher plants the amount of light required to darken photographic paper in 
one second. When dealing with lichens he adopted a more arbitrary standard, 
calculating as the unit the average amount of light that lichens would receive 
in entirely unshaded positions. He does not take account of the strength or 
duration of the light, and the conclusions he draws, though interesting and 
instructive, are only comparative. 

a. SUN LICHENS. The illumination of the Tundra lichens is reckoned 
by Wiesner as representing his unit of standard illumination. In the same 
category as these are included many of our most familiar lichens, which 
grow on rocks subject to the direct incidence of the sun's rays, such as, for 
instance, Parmelia conspersa, P. prolixa, etc. Physcia tcnella (Jiispidd) is also 
extremely dependent on light, and was never found by Wiesner under of 
full illumination. Dermatocarpon miniatum, a rock lichen with a peltate 
foliose thallus, is at its best from \ to of illumination, but it grows well in 
situations where the light varies in amount from I to ^ ? . Psora (Lecidea) 
lurida, with dark-coloured crowded squamules, grows on calcareous soil 
among rocks well exposed to the sun and has an illumination from I to ^, 
but with a poorer development at the lower figure. Many crustaceous rock 
lichens are also by preference sun-plants as, for instance, Verrucaria calciseda 
which grows immersed in calcareous rocks but with an illumination of. I 
to \\ in more shady situations, where the light had declined to ^, it was 
found to be less luxuriant and less healthy. 

Sun lichens continue to grow in the shade, but the thallus is then reduced 
and the plant is sterile. Zukal has made a list of those which grow best with 
a light-use of I to T \j, though they are also found not unfrequently in habitats 
where the light cannot be more than -$. Among these light-loving plants 
are the Northern Tundra species of Cladonia, Stereocanlon, Cetraria, Par- 
melia, Umbilicaria, and Gyrophora, as also Xanthoria parietina, Placodium 
elegans, P. murorum, etc., with some crustaceous species such as Lecanora 
atra, Haematomma ventosum, Diploschistes scruposus, many species of Leci- 
deaceae, some Collemaceae and some Pyrenolichens. 

Wiesner's conclusion is that the need of light increases with the lowering 
of the temperature, and that full illumination is of still more importance in 
the life of the plants when they grow in cold regions and are deprived of 
warmth: sun lichens are, therefore, to be looked for in northern or Alpine 
regions rather than in the tropics. 

b, COLOUR-CHANGES DUE TO LIGHT. Lichens growing in full sunlight 
frequently take on a darker hue. Cetraria islandica for instance in an open 
situation is darker than when growing in woods; C. aculeata on bare sand- 

1 Wiesner 1895. 
S. L. 16 



242 PHYSIOLOGY 

dunes is a deeper shade of brown than when growing entangled among 
heath plants. Parmelia saxatilis when growing on exposed rocks is fre- 
quently a deep brown colour, while on shaded trees it is normally a light 
bluish-grey. 

An example of colour-change due directly to light influences is given by 
Bitter 1 . He noted that the thallus of Parmelia obscurata on pine trees, and 
therefore subject only to diffuse light, grew to a large size and was of a light 
greyish-green colour marked by lighter-coloured lines, the more exposed 
lobes being always the most deeply tinted. In a less shaded habitat or in full 
sunlight the lichen was distinguished by a much darker colour, and the lobes 
were seamed and marked by blackish lines and spots. Bruce Fink 2 noted a 
similar development of dark lines on the thallus of certain rock lichens 
growing in the desert, more especially on Parmelia conspersa, Acarospora 
xanthophana and Lecanora muralis. He attributes a protective function to 
the dark colour and observes that it seemingly spreads from centres of con- 
tinued exposure, and is thus more abundant in older parts of the thallus. 
He contrasts this colouration with the browning of the tips of the fronds of 
fruticose lichens by which the delicate growing hyphae are protected from 
intense light. 

Gallic 3 finds that protection against too strong illumination is afforded 
both by white and dark colourations, the latter because the pigments catch 
the light rays, the former because it throws them back. The white colour 
is also often due to interspaces filled with air which prevent the penetration 
of the heat rays. 

A deepening of colour due to light effect often visible on exposed rock 
lichens such as Parmelia saxatilis is more pronounced still in Alpine and 
tropical species: the cortex becomes thicker and more opaque through the 
cuticularizing and browning of the hyphal membranes, and the massing of 
crystals on the lighted areas. The gonidial layer becomes, in consequence, 
more reduced, and may disappear altogether. Zukal 4 found instances of 
this in species of Cladonia, Parmelia, Roccella, etc. The thickened cortex 
acts also as a check to transpiration and is characteristic of desert species 
exposed to strong light and a dry atmosphere. 

Bitter 5 remarked the same difference of development in plants of Parmelia 
physodes : he found that the better lighted had a thicker cortex, about 20- 
30 jj, in depth, as compared with 15-22/4 or even only 12/u, in the greener 
shade-plants, and also that there was a greater deposit of acids in the more 
highly illuminated cortices, thus giving rise to the deeper shades of colour. 

Many lichens owe their bright tints to the presence of coloured lichen- 
acids, the production of which is strongly influenced by light and by clear 
air. Xanthoria parietina becomes a brilliant yellow in the sunlight: in the 

1 Bitter 1901, p. 465. 2 Fink 1909. 3 Gallic 1908. 4 Zukal 1896. s Bitter 1901. 



ILLUMINATION OF LICHENS 243 

shade it assumes a grey-green hue and yields only small quantities of 
parietin. Placodium elegans, normally a brightly coloured yellow lichen, 
becomes, in the strong light of the high Alps, a deep orange-red. Rhizo- 
carpon geographicum is a vivid citrine-yellow on high mountains, but is 
almost green at lesser elevations. 

c. SHADE LICHENS. Many species grow where the light is abundant 
though diffuse. Those on tree-trunks rarely receive direct illumination and 
may be generally included among shade-plants. Wiesner found that corti- 
colous forms of Parmelia saxatilis grew best with an illumination between 
and y^ of full light, and Pertusaria amara from ^ to ^j both of them could 
thrive from ^ to 3 ^, but were never observed on trees in direct light. Physcia 
ciliaris, which inhabits the trunks of old trees, is also a plant that prefers 
diffuse light. In warm tropical regions, lichens are mostly shade-plants: 
Wiesner records an instance of a species found on the aerial roots of a tree 
with an illumination of only -^. 

In a study of subterranean plants, Maheu 1 takes note of the lichens that 
he found growing in limestone caves, in hollows and clefts of the rocks, etc. 
A fair number grew well just within the opening of the caves; but species 
such as Cl. cervicornis, Placodium murorum and Xanthoria parietina ceased 
abruptly where the solar rays failed. Only a few individuals of one or two 
species were found to remain normal in semi-darkness: Opegrapha hapalea 
and Verrucaria muralis were found at the bottom of a cave with the thallus 
only slightly reduced. The nature of the substratum in these cases must 
however also be taken into account, as well as the light influences: lime- 
stone for instance is a more favourable habitat than gypsum ; the latter, being 
more readily soluble, provides a less permanent support. 

Maheu has recorded observations on growth in its relation to light in 
the case of a number of lichens growing in caves. 

Physcia obscura grew in almost total darkness; Placodium murorum 
within the cave had lost nearly all colour; Placodium variabile var. deep 
within the cave, sterile; Opegrapha endoleuca in partial obscurity; Verrucaria 
rupestris f. in total obscurity, the thallus much reduced and sterile; Verru- 
caria rupestris in partial obscurity, the asci empty; Homodium (Collema} 
granuliferum in the inmost recess of the cave, sterile, and the hyphae more 
spongy than in the open. 

Siliceous rocks in darkness were still more barren, but a few odd lichens 
were collected from sandstone in various caves : Cladonia squamosa, Parmelia 
perlata var. ciliata, Diploschistes scruposus, Lecidea grisella, Collema nigrescens 
and Leptogium lacerum. 

d. VARYING SHADE CONDITIONS. It has been frequently observed 
that on the trees of open park lands lichens are more abundant on the side 

1 Maheu 1906. 

16 2 



244 PHYSIOLOGY 

of the trunk that faces the prevailing winds. Wiesner 1 remarks that spores 
and soredia would more naturally be conveyed to that side; but there are 
other factors that would come into play: the tree and the branches frequently 
lean away from the wind, giving more light and also an inclined surface that 
would retain water for a longer period on the windward side 2 . Spores and 
soredia would also develop more readily in those favourable conditions. 

In forests there are other and different conditions: on the outskirts, 
whether northern or southern, the plants requiring more light are to be found 
on the side of the trunk towards the outside; in the depths of the forest, 
light may be reduced from ^^ to ^^, and any lichens present tend to be- 
come mere leprose crusts. Krempelhuber 3 has recorded among his Bavarian 
lichens those species that he found constantly growing in the shade: they 
are in general species of Collemaceae and Caliciaceae, several species of 
Peltigera (P. venosa, P. horizontalis and P. polydactyla) ; Solorina saccata ; 
Gyalecta Flotovii, G. cupularis; Pannaria microphylla, P. triptophylla, P. 
brunnea; Icmadophila aeruginosa, etc. 

B. EFFECT ON REPRODUCTIVE ORGANS 

In the higher plants, it is recognized that a certain light-intensity is 
necessary for the production of flowers and fruit. In the lower plants, such 
as lichens, light is also necessary for reproduction; it is a common observation 
that well-lighted individuals are the most abundantly fruited. In the higher 
fungi also, the fruiting body is more or less formed in the light. 

a. POSITION AND ORIENTATION OF FRUITS WITH REGARD TO LIGHT. 
There is an optimum of light for the fruits as well as for the thallus in each 
species of lichen : in most cases it is the fullest light that can be secured. 

Zukal 4 finds an exception to that rule in species of Peltigera: when 
exposed to strong sunlight, the lobes, fertile at the tips, curve over so that 
to some extent the back of the apothecium is turned to the light; with 
diffuse light, the horizontal position is retained and the apothecia face up- 
wards. In the closely allied genera Nephroma, Nephromium and Nephro- 
mopsis, the apothecia are produced on the back of the lobe at the extreme 
tip, but as they approach maturity the fertile lobes turn right back and they 
become exposed to direct illumination. In a well-developed specimen the 
full-grown fruits may thus become so prominent all over the thallus, that 
it is difficult to realize they are on reversed lobes. In one species of Cetraria 
(C. cucullatd) the rarely formed apothecia are adnate to the back of the lobe; 
but in that case the margins of the strap-shaped fronds are incurved and 
connivent, and the back is more exposed than the front. 

In Ramalina the frond frequently turns at a sharp angle at the point of. 
1 Wiesner 1895. a R. Paulson, ined. 3 Krempelhuber 1861. 4 Zukal 1896, p. in. 



ILLUMINATION OF LICHENS 245 

insertion of the apothecium which is thus well exposed and prominent; but 
Zukal 1 sees in this formation an adaptation to enable the frond to avoid 
the shade cast by the apothecium which may exceed it in width. In most 
lichens, however, and /especially in shade or semi-shade species, the repro- 
ductive organs are to be found in the best-lighted positions. 

b. INFLUENCE OF LIGHT ON COLOUR OF FRUITS. Lichen-acids are 
secreted freely in the apothecium from the tips of the paraphyses which give 
the colour to the disc, and as acid-formation is furthered by the sun's rays, 
the well-lighted fruits are always deeper in hue. The most familiar examples 
are the bright-yellow species that are rich in chrysophanic acid (parietin). 
Hedlund 2 has recorded several instances of varying colour in species of 
Micarea (Biatorina, etc.) in which very dark apothecia became paler in the 
shade. He also cites the case of two crustaceous species, Lecidea helvola and 
L. sulphnrella, which have white apothecia in the shade, but are darker in 
colour when strongly lighted. 

V. COLOUR OF LICHENS 

The thalli of many lichens, more especially of those associated with blue- 
green gonidia, are hygroscopic, and it frequently happens that any addition 
of moisture affects the colour by causing the gelatinous cell-walls to swell, 
thus rendering the tissues more transparent and the green colour of the 
gonidia more evident. As a general rule it is the dry state of the plant that 
is referred to in any discussion of colour. 

In the large majority of species the colouring is of a subdued tone soft 
bluish-grey or ash-grey predominating. There are, ho\vever, striking ex- 
ceptions, and brilliant yellow and white thalli frequently form a conspicuous 
feature of vegetation. Black lichens are rare, but occasionally the very dart 
brown of foliaceous species such as Gyrophora or of crustaceous species such 
as Verrncaria maura or Buellia atrata deepens to the more sombre hue. 

A. ORIGIN OF LICHEN-COLOURING 

The colours of lichens may be traced to several different causes. 
a. COLOUR GIVEN BY THE ALGAL CONSTITUENT. As examples may 
be cited most of the gelatinous lichens, Ephebaceae, Collemaceae, etc. which 
owe, as in Collema, their dark olivaceous-green appearance, when somewhat 
moist, to the enclosed dark -green gonidia, and their black colour, when dry, 
to the loss of transparency. When the thallus is of a thin texture as in 
Collema nigrescens, the olivaceous hue may remain constant. Leptogiutn 
Burgessii, another thin plant of the same family, is frequently of a purplish 
1 Zukal 1896. 2 Hedlund 1892, p. 11. 



246 PHYSIOLOGY 

hue owing to the purple colour of the gonidial Nostoc cells. The dull-grey 
crustaceous thallus of the Pannariaceae becomes more or less blue-green 
when moistened, and the same change has been observed in the Hymeno- 
lichens, Cora, etc. 

In Coenogonium, the alga is some species of Trentepohlia, a filamentous 
genus mostly yellow, which often gives its colour to the slender lichen 
filaments, the covering hyphae being very scanty. Other filamentous species, 
such as Usnea barbata, etc., are persistently greenish from the bright-green 
Protococcaceous cells lying near the surface of the thalline strands. Many 
of the furfuraceous lichens are greenish from the same cause, especially when 
moist, as are also the larger lichens, Physcia ciliaris, Stereocaulons, Cladonias 
and others. 

b. COLOUR DUE TO LICHEN-ACIDS. These substances, so characteristic 
of lichens, are excreted from the hyphae, and lie in crystals on the outer 
walls; they are generally most plentiful on exposed tissues such as the 
cortex of the upper surface or the discs of the apothecia. Many of these 
crystals are colourless and are without visible effect, except in sometimes 
whitening the surface, strikingly exemplified in Thamnolia vermicularis 1 ; 
but others are very brightly coloured. These latter belong to two chemical 
groups and are found in widely separated lichens 2 : 

1 . Derivatives of pul vinic acid which are usually of a bright-yellow colour. 
They are the colouring substance of Letharia vulpina, a northern species, not 
found in our islands, of Cetraria pinastri and C. juniperina* which inhabit 
mountainous or hilly regions. The crustaceous species, Lecidea lucida and 
Rhizocarpon geographicum, owe their colour to rhizocarpic acid. 

The brilliant yellow of the crusts of some species of Caliciaceae is due to 
the presence of the substance calycin, while coniocybic acid gives the greenish 
sulphur-yellow hue to Coniocybe furfuracea. Epanorin colours the hyphae 
and soredia of Lecanora epanora a citrine-yellow and stictaurin is the deep- 
yellow substance found in the medulla and under surface of Sticta aurata 
and 5. crocata. 

2. The second series of yellow acids are derivatives of anthracene. They 
include parietin, formerly described as chrysophanic acid, which gives the 
conspicuous colour to Xanthoriae<sx\& to various wall lichens; solorinic acid, 
the crystals of which cover the medullary hyphae and give a reddish-grey 
tone to the upper cortex of Solorina crocea, and nephromin which similarly 
colours the medulla of Nephromium lusitanicum a deep yellow, the colour of 
the general thallus being, however, scarcely affected. In this group must 
also be included the acids that cause the yellow colouring of the medulla in 
Parmelia subaurifera and the yellowish thallus of some Pertusariae. 

1 Zopf 1893. * Zopf 1907. 3 Zopf 1892. 



COLOUR OF LICHENS 247 

In many cases, changes in the normal colouring 1 are caused by the 
breaking up of the acids on contact with atmospheric or soil ammonia. 
Alkaline salts are thus formed which may be oxidized by the oxygen in 
the air to yellow, red, brown, violet-brown or even to entirely black humus- 
like products which are insoluble in water. These latter substances are 
frequently to be found at the base of shrubby lichens or on the under surface 
of leafy forms that are closely appressed to the substratum. 

c. COLOUR DUE TO AMORPHOUS SUBSTANCES. These are the various 
pigments which are deposited in the cell-walls of the hyphae. The only 
instance, so far as is known, of colours within the cell occurs in Baeomyces 
roseus, in which species the apothecia owe their rose-colour to oil-drops in 
the cells of the paraphyses, and in Lecidea coarctata where the spores are 
rose-coloured when young. In a few instances the colouring matter is 
excreted (Arthonia gregaria and Diploschistes ocellatus); but Bachmann 2 , 
who has made an extended study of this subject and has examined 120 
widely diversified lichens, found that with few exceptions the pigment was 
in the membranes. 

Bachmann was unable to determine whether the pigments were laid down 
by the protoplasm or were due to changes in the cell-wall. The middle 
layer, he found, was generally more deeply coloured than the inner one, 
though that was not universal. In other cases the outer sheath was the 
darkest, especially in cortices one to two cells thick such as those of Parmelia 
olivacea, P . fuliginosa and P. revoluta, and in the brown thick-walled spores 
of Physcia stellaris and of Rhizocarpon geographicum. Still another variation 
occurs in Parmelia tristis in which the dark cortical cells show an outer 
colourless membrane over the inner dark wall. 

The coloured pigments are mainly to be found in the superficial tissues, 
but if the thallus is split by areolation, as in crustaceous lichens, the internal 
hyphae may be coloured like those of the outer cortex wherever they are 
exposed. The hyphae of the gonidial layer are persistently colourless, but 
the lower surface and the rhizoids of many foliose lichens are frequently 
very deeply stained, as are the hypothalli of crustaceous species. 

The fruiting bodies in many different families of lichens have dark 
coloured discs owing to the abundance of dark-brown pigment in the para- 
physes. In these the walls, as determined by Bachmann, are composed 
generally of an inner wall, a second outer wall, and the outermost sheath 
which forms the middle lamella between adjacent cells. In some species 
the second wall is pigmented, in others the middle lamella is the one deeply 
coloured. The hymenium of many apothecia and the hyphae forming the 
amphithecium are often deeply impregnated with colour. The wall hyphae 

1 Knop 1872. 2 Bachmann 1890. 



248 PHYSIOLOGY 

of the pycnidia are also coloured in some forms; more frequently the cells 
round the opening pore are more or less brown. 

The presence of these coloured substances enables the cell-wall to resist 
chemical reactions induced by the harmful influences of the atmosphere or 
of the substratum. The darker the cell-wall and the more abundant the 
pigment, the less easily is the plant injured either by acids or alkalies. The 
coloured tips of the paraphyses thus give much needed protection to the 
long lived sporiferous asci, and the dark thalline tissues prevent premature 
rotting and decay. 

d. ENUMERATION OF AMORPHOUS PIGMENTS: 

1. Green. Bachmann found several different green pigments: "Lecidea- 
green," colouring red with nitric acid, is the dark blue-green or olive-green 
(smaragdine) of the paraphyses of many apothecia in the Lecideaceae, and 
may vary to a lighter blue; it appears almost black in thalline cells 1 . 
" Aspicilia-green " occurs in the thalline margin and sometimes in the 
epithecium of the fruits of species of Asp i cilia; it becomes a brighter green 
on the application of nitric acid. " Bacidia-green," also a rare pigment, 
becomes violet with the same acid; it is found in the epithecium of Bacidia 
muscorum and Bacidia acclinis (Lecideaceae). " Thalloidima-green " in the 
apothecia of some species of Biatorina is changed to a dirty-red by nitric 
acid and to violet by potash. Still another termed " rhizoid-green " gives 
the dark greenish colour to the rhizoids of Physciapulverulenta and P. aipolia 
and to the spores of some species of Physcia and Rhizocarpon. It becomes 
more olive-green with potash. 

2. Blue. A very rare colour in lichens, so far found in only a few species, 
Biatora (Lecidea} atrofusca, Lecidea sanguinaria and Aspicilia flavida f. 
coerulescens. It forms a layer of amorphous granules embedded in the outer 
wall of the paraphyses, becoming more dense towards the epithecium. A 
few granules are also present in the hymenium. 

3. Violet. " Arthonia-violet" as it is called by Bachmann is a constituent 
of the tissues of A rtlwnia gregaria, occurring in minute masses always near 
the cortical cells; it is distinct from the bright cinnabarine granules present 
in every part of the thallus. 

4. Red. Several different kinds of red have been distinguished: " Ur- 
ceolaria-red," visible as an interrupted layer on the upper side of the medulla 
in the thallus of Diploschistes ocellatus, a continental species with a massive, 
crustaceous, whitish thallus that shows a faint rose tinge when wetted. 
" Phialopsis-red " is confined to the epithecium of the brightly coloured 

1 A similar reaction with nitric acid is produced on the blue hypothalline hyphae of Placynthium 
nigrum. 



COLOUR OF LICHENS 249 

apothecia of Phialopsis rubra. " Lecanora-red," by which Bachmann desig- 
nates the purplish colour of the hymenium, is an unfailing character of 
Lecanora atra\ the colouring substance is lodged in the middle lamella of 
the paraphysis cells; it occurs also in Rhizocarpon geographicum and in Rk. 
viridiatrum\ it becomes more deeply violet with potash. M. C. Knowles 1 
noted the blue colouring of Rh. geographicum growing in W. Ireland near 
the sea and she ascribed it to an alkaline reaction. Two more rare pigments, 
" Sagedia-red " and " Verrucaria-red," are found in species of Verrucaria- 
ceae. These tinge the calcareous rocks in which the lichens are embedded 
a beautiful rose-pink. They are scarcely represented in our country. 

5. Brown. A frequent colouring substance, but also presenting several 
different kinds of pigment which may be arranged in two groups: 

(1) Substances with some characteristic chemical reaction. These 
are of somewhat rare occurrence: " Bacidia-brown " in the middle lamella 
of the paraphyses of Bacidia fuscorubella stains a clear yellow with acids 
or a violet colour with potash ; " Sphaeromphale-brown," which occurs 
in the perithecia and in the cortex of Staurothele clopismoides, becomes 
deep olive-green with potash, changing to yellow-brown on the application 
of sulphuric acid ; " Segestria-brown " in Porina lectissima changes to a 
beautiful violet colour with sulphuric acid, while " Glomellifera-brown," 
which is confined to the outer cortical cells of the upper surface of Parmelia 
glomellifera, becomes blue with nitric and sulphuric acids, but gives no re- 
action with potash. Rosendahl 2 confirmed Bachmann's discovery of this 
colour and further located it in corresponding cells of Parmelia prolixa and 
P. locarensis. 

(2) Substances with little or no chemical reaction. There is only 
one such to be noted: " Parmelia-brown," usually a very dark pigment, which 
is lodged in the outer membranes of the cells. It becomes a clearer colour 
with nitric acid, and if the reagent be sufficiently concentrated, some of the 
pigment is dissolved out. Some tissues, such as the lower cortex of some 
Panneliae, maybe so impregnated and hardened, that nothing short of boiling 
acid has any effect on the cells; membranes less deeply coloured and changed, 
such as the cortex of the Gyrophorae, become disintegrated with such drastic 
treatment. With potash the colour becomes darker, changing from a clear 
brown to olivaceous-brown or -green, or in some cases, as in a more faintly 
coloured epithecium, to a dirty-yellow, but the lighter colour produced there 
is largely due to the swelling up of the underlying tissues to which the potash 
penetrates readily between the paraphyses. 

" Parmelia-brown " is a colouring substance present in the dark epi- 
thecium and hypothecium of the fruits of many widely diverse lichens, and 

1 Knowles 1915. 2 Rosendahl 1907. 



250 PHYSIOLOGY 

in the cortical cells and rhizoids of many thalli. In some plants the thallus 
is brown both above and below, in others, as in Parmelia revoluta, etc. only 
the under surface is dark-coloured. 

e. COLOUR DUE TO INFILTRATION. There are several crustaceous lichens 
that are rusty-red, the colour being due to the presence of iron. These 
lichens occur on siliceous rocks of gneiss, granite, etc., and more especially 
on rocks rich in iron. Iron as a constituent of lichens was first demonstrated 
by John 1 in Ramalina fraxinea and R. calicaris. Grimbel 2 proved that the 
colour of rust lichens was due to an iron salt, and Molisch 3 by microscopic 
examination located minute granules of ferrous oxide as incrustations on 
the hyphae of the upper surface of the thallus. Molisch held that the rhizoids 
or penetrating hyphae dissolved the iron from the rocks by acid secretions. 
Rust lichens however grow on rocks that are frequently under water in which 
the iron is already present. 

Among " rusty " lichens are the British forms, Lecanora lacustris, the 
thallus of which is normally white, though generally more or less tinged 
with iron; it inhabits rocks liable to inundation. L. Dicksonii owes its fer- 
ruginous colour to the same influences. Lecidea contigua vax.flavicunda and 
L. confluens f. oxydata are rusty conditions of whitish-grey lichens. 

Nilson 4 found rusty lichens occurring frequently in the Sarak-Gebirge, 
more especially on glacier moraines where they were liable, even when un- 
covered by snow, to be flooded by water from the higher reaches. It is the 
thallus that is affected by the iron, rarely if ever are apothecia altered in 
colour. 

1 John 1819. 2 Grimbel 1856. 3 Molisch 1892. 4 Nilson 1907. 



251 













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Parmelia glome 


Parmelia-brown 



CHAPTER VI 

BIONOMICS 
A. GROWTH AND DURATION 

LICHENS are perennial plants mostly of slow growth and of long continuance ; 
there can therefore only be approximate calculations either as to their rate 
of increase in dimensions or as to their duration in time. A series of some- 
what disconnected observations have however been made that bear directly 
on the question, and they are of considerable interest. 

Meyer 1 was among the first to be attracted by this aspect of lichen life, 
and after long study he came to the conclusion that growth varied in 
rapidity according to the prevailing conditions of the atmosphere and 
the nature of the substratum ; but that nearly all species were very slow 
growers. He enumerates several, Lichen ( X anthoria) parietinus, L. (Par- 
melia) tiliaceus.L. (Rhizocarpori)geographicus, L.(Haematommd) ventosus,aind 
L. (Lecanoro) saxicolus, all species with a well-defined outline, which, after 
having attained some considerable size, remained practically unchanged for 
six and a half years, though, in some small specimens of foliose lichens, he 
noted, during the same period, an increase of one-fourth to one-third of their 
size in diameter. In one of the above crustaceous species, .. ventosus,the speci- 
men had not perceptibly enlarged in sixteen years, though during that time 
the centre of the thallus had been broken up by weathering and had again 
been regenerated. 

Meyer also records the results of culture experiments made in the open; 
possibly with soredia or with thalline scraps: he obtained a growth of 
X anthoria parietina (on wrought iron kept well moistened), which fruited in 
the second year, and in five years had attained a width of 5-6 lines (about 
i cm.) ; Lecanora saxicola growing on a moist rock facing south grew 4-7 lines 
in six and a half years, and bore very minute apothecia. 

Lindsay 2 quotes a statement that a specimen of Lobaria pulmonaria had 
been observed to occupy the same area of a tree after the lapse of half a 
century. Berkeley 3 records that a plant of Rhizocarpon geographicum remained 
in much the same condition of development during a period of twenty-five 
years. The latter is a slow grower and, in ordinary circumstances, it does 
not fruit till about fifteen years after the thallus has begun to form. Weddell 4 , 
also commenting on the long continuance of lichens, says there are crustaceous 
species occupying on the rock a space that might be covered by a five-franc 
piece, that have taken a century to attain that size. 

Phillips 5 on the other hand argues against the very great age of lichens, 
1 Meyer 1825, p. 44. 2 Lindsay 1856. 3 Berkeley 1857. * Weddell 1869. 5 Phillips 1878. 



GROWTH AND DURATION 253 

and suggests 20 years as a sufficient time for small plants to establish them- 
selves on hard rocks and attain full development. He had observed a small 
vigorous plant of Xanthoria parietina that in the course of five years had 
extended outwards to double its original size. The centre then began to 
break up and the whole plant finally disappeared. 

Exact measurements of growth have been made by several observers. 
Scott Elliot 1 found that a Pertusaria had increased about half a millimetre 
from the ist February to the end of September. Vallot 2 kept under obser- 
vation at first three then five different plants of Parmelia saxatilis during a 
period of eight years : the yearly increase of the thallus was half a centimetre, 
so that specimens of twenty centimetres in breadth must have been growing 
from forty to fifty years. 

Bitter's 3 observations on Parmelia physodes agree in the main with those 
of Vallot: the increase of the upper lobes during the year was 3-4 mm. In 
a more favourable climate Heere found that Parmelia caperata (Fig. 49) on 
a trunk ofAescu/us in California had grown longitudinally 1*5 cm. and trans- 
versely i cm. The measurements extended over a period of seven winter 
months, five of them being wet and therefore the most favourable season of 
growth. In warm regions lichens attain a much greater size than in tem- 
perate or northern countries, and growth must be more rapid. 

A series of measurements was also made by Heere 4 on Ramalina reti- 
culata (Fig. 64), a rapid growing tree-lichen, and one of the largest American 
species. The shorter lobes were selected for observation, and were tested 
during a period of seven months from September to May, five of the months 
being in the wet season. ' There was great variation between the different 
lobes but the average increase during that period was 41 per cent. 

Krabbe 5 took notes of the colonization of Cladonia rangiferina (Fig. 127) 
on burnt soil : in ten years the podetia had reached a height of 3 to 5 cm., 
giving an annual growth of about 3-5 mm. It is not unusual to find speci- 
mens in northern latitudes 18 inches long (50 cm.), which, on that computa- 
tion, must have been 100 to 160 years old; but while increase goes on at the 
apex of the podetia, there is constant perishing at the base of at least as 
much as half the added length and these plants would therefore be 200 or 
300 years old. Reinke 6 indeed has declared that apical growth in these 
Cladina species may go on for centuries, given the necessary conditions of 
good light and undisturbed habitat. 

Other data as to rate of growth are furnished by Bonnier 7 in the account 
of his synthetic cultures which developed apothecia only after two to three 
years. The culture experiments of Darbishire 8 and Tobler 9 with Cladonia 
soredia are also instructive, the former with synthetic spore- and alga-cultures 

1 Scott Elliot 1907. * Vallot 1896. 3 Bitter 1901. * Heere 1904. s Krabbe 1891, p. 131. 
6 Reinke 1894, p. 18. 7 Bonnier, see p. 29. 8 Darbishire, see p. 148. 9 Tobler, see p. 148. 



254 BIONOMICS 

having obtained a growth of soredia in about seven months; the latter, 
starting with soredia, had a growth of well-formed squamules in nine months. 

It has been frequently observed that abundance of moisture facilitates 
growth, and this is nowhere better exemplified than in crustaceous soil- 
lichens. Meyer found that on lime-clay soil which had been thrown up from 
a ditch in autumn, lichens such as Gyalecta geoica were fully developed the 
following summer. He gives an account also of another soil species, Verru- 
caria (Thrombium) epigaea, which attained maturity during the winter half 
of the year. Stahl 1 tells us that Thelidium minutulum, a pyrenocarpous soil- 
lichen, with a primitive and scanty thallus, was cultivated by him from spore 
to spore in the space of three months. Such lichens retain more of the 
characteristics of fungi than do those with a better developed thallus. Rapid 
colonization by a soil-lichen was also observed in Epping Forest by Paulson 2 . 
In autumn an extensive growth of Lecidea uliginosa covered as if with a dark 
stain patches of soil that had been worn bare during the previous spring. 
The lichen had reached full development and was well fruited. 

These facts are quite in harmony with other observations on growth 
made on Epping Forest lichens. The writers 3 of the report record the finding 
of " fruiting lichens overspreading decaying leaves which can scarcely have 
lain on the ground more than two or three years; others growing on old 
boots or on dung and fruiting freely; others overspreading growing mosses." 
They also cite a definite instance of a mass of concrete laid down in 1903 
round a surface-water drain which in 1910 seven years later was covered 
with Lecanora galactina in abundant fruit; and of another case of a Portland 
stone garden -ornament, new in 1904, and, in 1910, covered with patches of 
a fruiting Verrucaria (probably V. nigrescens}. Both these species, they add, 
have a scanty thallus and generally fruit very freely. 

A series of observations referring to growth and "ecesis" or the spreading 
of lichens have been made by Bruce Fink 4 over a period of eight years. His 
aim was mainly to determine the time required for a lichen to re-establish 
itself on areas from which it had been previously removed. Thus a quadrat 
of limestone was scraped bare of moss and of Leptogium lacerum, except 
for bits of the moss and particles of the lichen which adhered to the 
rock, especially in depressions of the surface. After four years, the moss 
was colonizing many small areas on which grew patches of the lichen 2 to 
10 mm. across. Very little change occurred during the next four years. 

Numerous results are also recorded as to the rate of growth, the average 
being i cm. per year or somewhat under. The greatest rate seems to have 
been recorded for a plant of Peltigera canina growing on " a mossy rock 
along a brook in a low moist wood, well-shaded." A plant, measuring 10 
by 14 cm., was deprived of several large apothecia. The lobes all pointed 
1 Stahl 1877, p. 34. 2 Paulson 1918. 3 Paulson and Thompson 1913. 4 Fink 1917. 



GROWTH AND DURATION 255 

in the same direction, and the plant increased 175 cm. in one year. Two 
other plants, deprived of their lobes, regenerated and increased from 2 and 
5 cm. respectively to 3^5 and 6 cm. No other measurements are quite so 
high as these, though a plant of Parmelia caperata (sterile), measuring from 
I to 2 cm. across, reached in eight years a dimension of 10 by 13 cm. Other 
plants of the same species gave much slower rates of increase. A section of 
railing was marked bearing minute scattered squamules of Cladonia pityrea. 
After two years the squamules had attained normal size and podetia were 
formed 2 to 4 mm. long. 

Several areas of Verrucaria muralis were marked and after ten months 
were again measured; the largest plants, measuring 2*12 by 2^4 cm. across, 
had somewhat altered in dimensions and gave the measurements 2'2 by 
3 cm. Some crustose species became established and produced thalli and 
apothecia in two to eight years. Foliose lichens increased in diameter from 
'3 to 3'5 cm - P er y ear - So far as external appearance goes, apothecia are 
produced in one to eight years; it is concluded that they require four to 
eight years to attain maturity in their natural habitats. 

B. SEASON OF FRUIT FORMATION 

The presence of apothecia (or perithecia) in lichens does not always 
imply the presence of spores. In many instances they are barren, the spores 
having been scattered or not yet matured ; the disc in these cases is composed 
of paraphyses only, with possible traces of asci. In any month of the year, 
however, some lichens may be found in fruit. 

Baur 1 found, for instance, that Parmelia acetabulum developed carpogonia 
the whole year round, though somewhat more abundantly in spring and 
autumn. Pertusaria communis similarly has a maximum period of fruit- 
formation at these two seasons. This is probably true of tree-lichens 
generally: in summer the shade of the foliage would inhibit the formation 
of fruits, as would the extreme cold of winter ; but were these conditions 
relaxed spore-bearing fruits might be expected at any season though perhaps 
not continuously on the same specimen. 

An exception has been noted by Baur in Pyrenula nitida, a crustaceous 
tree Pyrenolichen. He found carpogonia only in February and April, and 
the perithecia matured in a few weeks, presumably at a date before the trees 
were in full leaf; but even specimens of Pyrenula are not unusual in full 
spore-bearing conditions in the autumn of the year. 

To arrive at any true knowledge as to the date and duration of spore 
production, it would be necessary to keep under observation a series of one 
species, examining them microscopically at intervals of a few weeks or months 

1 Baur 1901. 



256 BIONOMICS 

and noting any conditions that might affect favourably or unfavourably the 
reproductive organs. A comparison between corticolous and saxicolous 
species would also be of great interest to determine the influence of the 
substratum as well as of light and shade. But in any case it is profitable to 
collect and examine lichens at all seasons of the year, as even when the 
bulk of the spores is shed, there may remain belated apothecia with a few 
asci still intact. 

C. DISPERSAL AND INCREASE 

The natural increase of lichen plants may primarily be sought for in the 
dispersal of the spores produced in the fruiting-bodies. These are ejected, 
as in fungi, by the pressure of the paraphyses on the mature ascus. The 
spores are then carried away by wind, water, insects, etc. In a few lichens 
gonidia are enclosed in the hymen ium and are ejected along with the spores, 
but, in most, the necessary encounter with the alga is as fortuitous, and 
generally as certain, as the pollination of anemophilous flowers. A case of 
dispersal in Sagedia microspora has been described by Miyoshi 1 in which 
entire fruits, small round perithecia, were dislodged and carried away 
by the wind. The addition of water caused them to swell enormously and 
brought about the ejection of the spores. Areas covered by the thallus 
are also being continually enlarged by the spreading growth of the hypo- 
thallus. 

a. DISPERSAL OF CRUST ACEOUS LICHENS. These lichens are distributed 
fairly equally on trees or wood (corticolous) and on rocks (saxicolous). Some 
species inhabit both substrata. As regards corticolous lichens that live on 
smooth bark such as hazel or mountain-ash, the vegetative body or thallus 
is generally embedded beneath the epidermis of the host. Soredia are absent 
and the thallus is protected from dispersal. In these lichens there is rather 
an abundant and constant formation of apothecia or perithecia. 

Other species that affect rugged bark and are more superficial are less 
dependent on spore production. The thallus is either loosely granular, or is 
broken up into areolae. The areolae are each a centre of growth, and with 
an accession of moisture they swell up and exert pressure on each other. 
Parts of the thallus thus become loosened and are dislodged and carried 
away. If anchored on a suitable substratum they grow again to a complete 
lichen plant. Sorediate lichens are dependent almost wholly on these bud- 
like portions for increase in number ; soredia are easily separated from 
the parent plant, and easily scattered. Darbishire 2 noted frequently that 
small Poduridae in moving over the surface of Pertusaria amara became 
powdered with soredia and very evidently took a considerable part in the 
dissemination of the species. 

1 Miyoshi 1901. 2 Darbishire 1897, p. 657. 



DISPERSAL AND INCREASE 257 

Crustaceous rock lichens are rarely sorediate, but they secure vegetative 
propagation l by the dispersal of small portions of the thallus. The thalli most 
securely attached are cracked into small areolae which, by unequal growth, 
become very soon lop-sided, or, by intercalary increase, form little warts and 
excrescences on their surface. These irregularities of development give rise 
to more or less tension which induces a loosening of the thallus from the 
substratum. Weather changes act similarly and gradually the areolae are 
broken off. Loosening influence is also exercised by the developing fruits, 
the expanding growth of which pushes aside the neighbouring tissues. Wind 
or water then carries away the thalline particles which become new centres 
of growth if a suitable substratum is reached. 

b. DISPERSAL OF FOLIOSE LICHENS. It is a matter of common obser- 
vation that, in foliose lichens where fruits are abundant, there are few or no 
soredia and vice versa. In either case propagation is ensured. In addition 
to these obvious methods of increase many lichens form isidia, outgrowths 
from the thallus which are easily detached. Bitter 2 considers for instance 
that the coralloid branchlets, which occur in compact tufts on the thallus of 
Uinbilicaria pustulata, are of immense service as organs of propagation. 
Apothecia and pycnidia are rarely present in that species, and the plant 
thus falls back on vegetative production. Slender crisp thalline outgrowths, 
easily separable, occur also on the edges of lobes, as in species of Peltigera, 
Platysvia, etc. 

Owing to the gelatinous character of lichen hyphae, the thallus quickly 
becomes soft with moisture and is then easily torn and distributed by wind, 
animals, etc. The action of lichens on rocks has been shown to be of a 
constantly disintegrating character, and the destruction of the supporting 
rock finally entails the scattering of the plant. This cause of dispersal is 
common to both crustaceous and foliose species. The older central parts of 
a lichen may thus have disappeared while the areolae on lobes of the cir- 
cumference are still intact and in full vigour. 

As in crustaceous lichens the increase in the area of growth may take 
place by means of the lichen mycelium which, originating from the rhizinae 
in contact with the substratum, spreads as a hypothallus under the shelter 
of the lobes and far beyond them. When algae are encountered a new lobe 
begins to form. The process can be seen perhaps most favourably in 
lichens on decaying wood which harbours moisture and thus enables the 
wandering hyphae to retain life. 

c. DISPERSAL OF FRUTICOSE LICHENS. Many of these lichens are 
abundantly fruited; in others soralia are as constantly developed. Species 
of Usnea, Alectoria, Ramalina and many Cladoniae are mainly propagated 

1 Beckmann 1907. '* Bitter 1899. 

S. L. I 7 



258 BIONOMICS 

by soredia. They are all peculiarly liable to be broken and portions of the 
thallus scattered by the combined action of wind and rain. 

Peirce 1 found that Ramalina reticulata (Fig. 65), of which the fronds are 
an open network, was mainly distributed by the tearing of the lichen in high 
wind. This takes place during the winter rains, when not only the lichen is 
wet and soft in texture, but when the deciduous trees are bare of leaves, at 
a season, therefore, when the drifting thalline scraps can again catch on to 
branch or stem. A series of observations on the dispersal of forms of long 
pendulous Usneas was made by Schrenk 2 . In the Middle and North Atlantic 
States of America these filamentous species rarely bear apothecia. The 
high winds break and disperse them when they are in a wet condition. They 
generally grow on Spruces and Firs, because the drifting filaments are more 
easily caught and entangled on short needles. The successive wetting and 
drying causes them to coil and uncoil, resulting in a tangle impossible to 
unravel, which holds them securely anchored to the support. 

D. ERRATIC LICHENS 

In certain lichens, there is a tendency for the thallus to develop excres- 
cences of nodular form which easily become free and drift about in the wind 
while still living and growing. They are carried sometimes very long distances, 
and fall in thick deposits over localities far from their place of origin. The 
most famous instance is the " manna lichen," Lecanora esculenta, which has 
been scientifically examined and described by Elenkin 3 . He distinguishes 
seven different forms of the species: f. esculenta, f. affznis, f. alpina, and 
f. fnttiadosa-foliacea which are Alpine lichens, the remainder, f. desertoides, 
f. foliacea and f. esculenta-tarquina, grow on the steppes or in the desert 4 . 

Elenkin 3 adds to the list of erratic lichens a variety of Parmelia mollius- 
cula along with P. ryssolea from S. Russia, from the Asiatic steppes and 
from Alpine regions. Mereschkovsky 5 has also recorded from the Crimea 
Parmelia vagans, probably derived from P. conspersa f. vaga (f. nov.). It 
drifts about in small rather flattened bits, and, like other erratics, it never 
fruits. 

Meyer 6 long ago described the development of wandering lichens : scraps 
that were torn from the parent thallus continued to grow if there were 
sufficient moisture, but at the same time undergoing considerable change in 
appearance. The dark colour of the under surface disappears in the frequently 
altered position, as the lobes grow out into narrow intermingling fronds 
iorming a more or less compact spherical mass ; the rhizoids also become 
modified and, if near the edge, grow out into threadlike structures which 

Peirce 1898. 2 Schrenk 1898. 3 Elenkin 1901. 4 See Chap. X. 

5 Mereschkovsky 1918. 6 Meyer 1825, p. 44. 



ERRATIC LICHENS 



259 



bind the mass together. Meyer says that " wanderers " have been noted as 

belonging to P annelid acetabuluin, Platysma glaucum and Anaptychiaciliaris. 

The most notable instance in Britain of the " erratic " habit is that of 

Parmelia revoluta var. concentrica (Fig. 121), first found on Melbury Hill 











li" 




Fig. 121. Parmelia revoluta var. concentrica Cromb. a, plant on flint with detached fragment; 
b, upper surface of three specimens ; c, three specimens as found on chalk downs ; d, speci 
in section showing central cavity (S. H., Photo.}. 

17 2 



2 6o BIONOMICS 

near Shaftesbury, Dorset, and described as " a spherical unattached lichen 
which rolls on the exposed downs." It has recently been observed on the 
downs near Seaford in Sussex, where, however, it seems to be confined to a 
small area about eight acres in extent which is exposed to south-west winds. 
The lichen is freely distributed over this locality. To R. Paulson and Somer- 
ville Hastings 1 we owe an account of the occurrence and origin of the revo- 
luta wanderers. The specimens vary considerably in shape and size, and 
measure from I to 7 cm. in longest diameter. Very few are truly spherical, 
some are more or less flattened and many are quite irregular. The revolute 
edges of the overlapping lobes give a rough exterior to the balls, which 
thereby become entangled amongst the grass, etc., and movement is impeded 
or prevented, except in very high winds. Crombie 2 had suggested that the 
concentric plant originated from a corticolous habitat, but no trees are near 
the Seaford locality. Eventually specimens were found growing on flints in 
the immediate neighbourhood. While still on the stone the lichen tends 
to become panniform, a felt of intermingling imbricate lobes is formed, 
portions of which, in time, become crowded out and dislodged. When 
scattered over the ground, these are liable to be trampled on by sheep or 
other animals and so are broken up; each separate piece then forms the 
nucleus of new concentric growth. 

Crombie 2 observed at Braemar, drifting about on the detritus of Morrone, 
an analogous structure in Parmelia omphalodes. He concluded that nodular 
excrescences of the thallus had become detached from the rocks on which 
the lichen grew; while still attached to the substratum Parmelia omphalodes 
and the allied species, P. saxatilis, form dense cushion-like masses. 

E. PARASITISM 

a. GENERAL STATEMENT. The parasitism of Strigula complanata, an 
exotic lichen found on the leaves of evergreen trees, has been already 
described 3 ; Dufrenoy 4 records an instance of hyphae from a Parmelia thallus 
piercing pine-needles through the stomata and causing considerable injury. 
Lichen hyphae have attacked and destroyed the protonemata of mosses. 
Cases have also been recorded of Usnea and Ramalina penetrating to the living 
tissue of the tree on which they grew, and there may be other similar para- 
sitisms ; but these exceptions serve to emphasize the independent symbiotic 
growth of lichens. 

There are however some lichens belonging to widely diverse genera that 
have retained, or reverted to, the saprophytic or parasitic habit of their fungal 
ancestors, though the cases that occur are generally of lichens preying on 

1 Paulson and Somerville Hastings 1914. 2 Crombie 1872. 8 See p. 35. 

* Dufrenoy 1918. 



PARASITISM 261 

other lichens. The conditions have been described as those of " antagonistic 
symbiosis " when one lichen is hurtful or fatal in its action on the other, and 
as " parasymbiosis " when the association does little or no injury to the host. 
The parasitism of fungi on lichens, though falling under a different category, 
in many instances exhibits features akin to parasymbiosis. 

The parasitism of fungus on fungus is not unusual; there are instances 
of its occurrence in all the different classes. In the Phycomycetes there are 
genera wholly parasitic on other fungi such as Woronina and other Chytri- 
diaceae ; Piptocephahts, one of the Mucorini, is another instance. Cicinnobolus, 
one of the Sphaeropsideae, preys on Perisporiae ; a species of Cordyceps is 
found on Elaphomyces, and Orbilia coccinella on Polyporus\ while among 
Basidiomycetes, Nyctalis, an agaric, grows always on Russula. 

There are few instances of lichens rinding a foothold on fungi, for the 
simple reason that the latter are too short lived. On the perennial Polyporeae 
a few have been recorded by Arnold 1 , but these are not described as doing 
damage to the host. They are mostly species of Lecidea or of allied genera. 
Kupfer 2 has also listed some 15 different lichens that he found on Lenzites sp. 

b. ANTAGONISTIC SYMBIOSIS. In discussing the nutrition of lichens 3 
note has been taken of the extent to which some species by means of enzymes 
destroy the thallus of other lichens in their vicinity and then prey on the 
dead tissues. A constantly cited 4 example is that of Lecanora atriseda which 
in its early stages lives on the thallus of Rhizocarpon geographicum inhabiting 
mountain rocks. A detailed examination of the relationship between these 
two plants was made by Malme and later by Bitter 5 . Both writers found 
that the Lecanora thallus as it advanced caused a blackening of the Rhizo- 
carpon areolae, the tissues of which were killed by the burrowing slender 
filaments of the Lecanora, easily recognized by their longer cells. The invader 
thereafter gradually formed its own medulla, gonidial layer and cortex right 
over the surface of the destroyed thallus. Lecidea insularis (L. intumescens) 
similarly takes possession of and destroys the thallus of Lecanora glaucoma 
and Malme 4 strongly suspects that Bnellia verruculosa and B. aethalea may 
be living on the thallus of Rhizocarpon distinctum with which they are 
constantly associated. 

Other cases of facultative parasitism have been studied by Hofmann 6 , 
more especially three different species, Lecanora dispersa, Lecanora sp. and 
Parmelia hyperopta, which were found growing on the thick foliose thailus 
of Dermatocarpon miniatum. These grew, at first independently, on a wall 
along with many examples of Endocarpon on to which they spread as oppor- 
tunity offered. The thallus of the latter was in all cases distorted, the area 
occupied by the invaders being finally killed. The attacking lichens had 

1 Arnold 1874. - Kupfer 1894. 3 See p. 236. * Malme 1895. 

5 Bitter 1899. 6 Hofmann 1906. 



262 BIONOMICS 

benefited materially by the more nutritive substratum : their apothecia were 
more abundant and their thallus more luxuriant. The gonidia especially 
had profited; they were larger, more brightly coloured, and they increased 
more freely. Hoffmann offers the explanation that the strain on the algae of 
providing organic food for the hyphal symbiont was relaxed for the time, 
hence their more vigorous appearance. 

Arthonia subvarians is always parasitic on the apothecia of Lecanora 
galactina, and Almquist 1 discovered that the hymenium of the host alone is 
injured, the hypothecium and excipulum being left intact. 

The " parasitism " of Pertusaria globulifera on Parmelia perlata and 
P.physodes, as described by Bitter 2 , may also be included under antagonistic 
symbiosis. The hyphae pierce the Parmdia thallus, break it up and gradually 
absorb it. Chemical as well as mechanical influences are concerned in the 
work of destruction as both the fungus and the alga of the victim are dissolved. 
Lecanora tartarea already dealt with as a marauding lichen 3 over decaying 
vegetation may spread also to living lichens. Fruticose soil species, such as 
Cetraria aculeata and others, die from the base and the Lecanora gains 
entrance to their tissues at the decaying end which is open. 

Arnold 4 speaks of these facultative parasites that have merely changed 
their substratum as pseudo-parasites, and he gives a list of instances of such 
change. In many cases it is rather the older thalli that are taken possession 
of, and, in nearly every case, the invader is some crustaceous species. The 
plants attacked are generally ground lichens or more particularly those that 
inhabit damp localities, such as Peltigera or Cladonia or certain bark lichens. 
Drifting soredia or particles of a lichen would easily take hold of the host 
thallus and develop .in suitable conditions. To give a few of the instances 
observed, there have been found, by Arnold, Crombie and others: 

on Peltigera canina: Callopisma cerina, Rinodina turfacea var., Bilimbia 
obscurata and Lecanora aurella; 

on Peltigera aphthosa: Lecidea decolorans; 

on Cladoniae: Bilimbia microcarpa, Bacidia Beckhausii and Urceolaria 
scruposa, etc. 

Urceolaria (Diploschistes) has a somewhat bulky crustaceous thallus which 
may be almost evanescent in its semi-parasitic condition, the only gonidia 
retained being in the margin of the apothecia. Nylander 5 found isolated 
apothecia growing vigorously on Cladonia squamules. 

Hue 6 describes Lecanora aspidophora f. errabunda, an Antarctic lichen, as 
not only a wanderer but as a "shameless robber." It is to be seen everywhere 
on and about other lichens, settling small glomeruli of apothecia here and 

1 Almquist 1880. 2 Bitter 1899. 3 See p. 237. 4 Arnold 1874. 

6 Nylander 1852. 6 Hue 1915. 



PARASITISM 263 

there on the thallus of Umbilicariae or between the areolae of Buelliac, and 
always too vigorous to be ousted from its position. 

Bacidia flavovirescens has been regarded by some lichenologists 1 as a 
parasite on Baeomyces, but recent work by Tobler 2 seems to have proved 
that the bright green thallus is that of the Bacidia. 

c. PARASYMBIOSIS. There are certain lichens that are obligative parasites 
and pass their whole existence on an alien thallus. They may possibly have 
degenerated from the condition of facultative parasitism as the universal 
history of parasitism is one of increased dependence on the host, and of 
growing atrophy of the parasite, but, in the case of lichens, there is always 
the peculiar symbiotic condition to be considered : the parasite produces its 
own vigorous hyphae and normal healthy fruits, it often claims only a share 
of the carbohydrates manufactured by the gonidia. The host lichen is not 
destroyed by this parasymbiosis though the tissues are very often excited 
to abnormal growth by the presence of the invading organism. 

Lauder Lindsay 3 was one of the first to study these "microlichens" as 
he called them, and he published descriptions of those he had himself 
observed on various hosts. He failed however to discriminate between lichens 
and parasitic fungi. It is only by careful research in each case that the 
affinity to fungi or to lichens can be determined; very frequently the whole 
of them, as possessing no visible thallus, have been classified with fungi, but 
that view ignores the symbiosis that exists between the hyphae of the 
parasite and the gonidia of the host. 

Parasitic lichens are rather rare on gelatinous thalli ; but even among 
these, a few instances have been recorded. Winter 4 has described a species 
of LeptorapJtis, the perithecia of which are immersed in the thallus of Physma 
franconicum. The host is wholly unaffected by the presence of the parasite 
except for a swelling where it is situated. The foreign hyphae are easily 
distinguishable; they wander through the thallus of the host with their free 
ends in the mucilage of the gonidial groups from which they evidently 
extract nourishment. Species of the lichen genus Obryznm are also parasitic 
on gelatinous lichens. 

The parasitic genus Abrothallus* has been the subject of frequent stud}-. 
There are a number of species which occur as little black discs on various 
thalli of the large foliose lichens. They were first of all described as parasitic 
fungi, later Tulasne 6 affirmed their lichenoid nature as proved by the struc- 
ture, consistence and long duration of the apothecia. Lindsay 7 wrote a 
monograph of the genus dealing chiefly with Abrothallus Smithii (Buellia 
P armeliarunt) and A. oxysporus, with their varieties and forms that occur on 

1 Th. Fries 1874, p. 343. "- Tobler 191 1 2 . 3 Lindsay i86 9 2 . 4 Winter 1877. 

5 Abrothallus has been included in the lichen genus Buellia. 6 Tulasne 1852. 

7 Lindsay 1856. 



264 BIONOMICS 

several different hosts. In some instances the thallus is apparently quite 
unaffected by the presence of Abrothallus, in others, as in Cetraria glauca, 
there is considerable hypertrophy produced, the portion of the thallus on 
which the parasites are situated showing abnormal growth in the form of 
swellings or pustules which may be regarded as gall-formations. Crombie 1 
points this out in a note on C. glauca var. ampullacea, figured first by 
Dillenius, which is merely a swollen condition due to the presence of 
Abrothallus. 

The internal structure and behaviour of Abrothallus has more recently 
been followed in detail by Kotte 2 . He recognized a number of different 
species growing on various thalli of Parmelia and Cetraria, but Abrothallus 
Cetrariae was the only one that produced gall-formation. The mycelium of 
the parasite in this instance penetrates to the medulla of the host lichen as 
a loose weft of hyphae which are divided into more or less elongate cells. 
These send out side branches, which grow towards the algal cells, and by 
their short-celled filaments clasp them exactly in the same way as do the 
normal lichen hyphae. Thus in the neighbourhood of the parasite an algal 
cell may be surrounded by the hyphae not only of the host, but also by 
those of Abrothallus. The two different hyphae can generally be distin- 
guished by their reaction to iodine: in some cases Abrothallus hyphae take 
the stain, in others the host hyphae. In addition to apothecia, spermogonia 
or pycnidia are produced, but in one of the species examined by Kotte, 
Abrothallus Peyritschii on Cetraria caperata, there was no spermogonial 
wall formed. The hyphae also penetrate the host soredia or isidia, so that 
on the dispersal of these vegetative bodies the perpetuation of both organisms 
is secured in the new growth. 

Abrothallus draws its organic food from the gonidia in the same way as 
the host species, and possibly the parasitic hyphae obtain also water and 
inorganic food along with the host hyphae. They have been traced down 
to the rhizinae and may even reach the hypothallus, but no injury to the 
host has been detected. It is a case of joint symbiosis and not of parasitism. 
Microscopic research has therefore justified the inclusion of these and other 
forms among lichens. 

d. PARASYMBIOSIS' OF FUNGI. There occur on lichens, certain parasites 
classed as fungi which at an early stage are more or less parasymbionts of 
the host ; as growth advances they may become parasitic and cause serious 
damage, killing the tissues on which they have settled. 

Zopf 3 found several instances of such parasymbiosis in his study of 

fungal parasites, such as Rhymbocarpus punctiformi s, a minute Discomycete 

which inhabits the thallus of Rhizocarpon geographicum. By means of 

staining reagents he was able to trace the course of the parasitic hyphae, 

1 Crombie 1894. 2 Kotte 1910. 3 Zopf 1896. 



PARASITISM 265 

and found that they travelled towards the gonidia and clasped them lichen- 
wise without damaging them, since these remained green and capable of 
division. At no stage was any harm caused to the host by the alien 
organism. Another instance he observed was that of Conida rubescens on 
the thallus of Rhizocarpon epipolium. By means of fine sections through the 
apothecia of Conida and the thallus of the host, he proved the presence of 
numerous gonidia in the subhymenial tissue, these being closely surrounded 
by the hyphae of the parasite, and entirely undamaged : they retained their 
green colour, and in size and form were unchanged. Zopf 1 at first described 
these parasites as fungi though later 1 he allows that they may represent 
lower forms of lichens. 

Tobler 2 has added two more of these parasymbiotic species on the border 
line between lichens and fungi, similar to those described by Zopf. One of 
these, Phacopsis vulpina, belonging to the fungus family Celidiaceae, is 
parasitic on Letharia vulpina. The fronds of the host plant are considerably 
altered in form by its presence, being more branched and curly. Where 
the parasite settles a swelling arises filled with its hyphae, and the host 
gonidia almost disappear from the immediate neighbourhood, only a few 
"nests" being found and these very mucilaginous. These nests as well as 
single gonidia are surrounded by Phacopsis hyphae which have gradually 
displaced those of the Letharia thallus. The gonidia are excited to division 
and increase in number on contact with either lichen or fungus hyphae, but 
in the latter case the increase is more abundant owing doubtless to a more 
powerful chemical irritant in the fungus. As development advances, the 
Phacopsis hyphae multiply to the exclusion of both lichen hyphae and 
gonidia from the area of invasion. Finally the host cortex is split, the 
fungus bursts through, and the tissue beneath the parasite becomes brown 
and dead. Phacopsis begins as a "parasymbiont," then becomes parasitic, 
and is at last saprophytic on the dead cells. The hyphae travel down into 
the medulla of the host and also into the soredial outgrowths, and are 
dispersed along with the host. The effect of Verrucula on the host thallus 
may also be cited 3 . 

Tobler gives the results of his examination of still another fungus, Kar- 
schia destructans. It becomes established on the thallus of Chaenotheca 
cJnysoceptiala and its hyphae gradually penetrate down to the underlying 
bark (larch). The lichen thallus beneath the fungus is killed, but gonidia in 
the vicinity are sometimes clasped : Karschia also is thus a parasymbiont, 
then a parasite, and finally a saprophyte. 

Elenkin 4 describes certain fungi which to some extent are parasymbionts. 
One of these, Conidclla urceolata n.sp., grew on forms of Lecanora esculenta. 
The other, a stroma-forming species, had invaded the thallus of Parmelia 

1 Zopf 1898, p. 249. 2 Tobler 191 1 2 . 3 See p. 276. 4 Elenkin 1901-. 



266 BIONOMICS 

molliuscula, where it caused gall-formation. As the growth of the gall was 
due to the co-operation of the lichen gonidia, the fungus must at first have 
been a parasymbiont. Only dead gonidia were present in the stroma; prob- 
ably they had been digested by the parasite. Because of the stroma Elenkin 
placed the fungus in a new genus, Trematosphaeriopsis. 

e. FUNGI PARASITIC ON LICHENS. A solution or extract of lichen 
thallus is a very advantageous medium in which to grow fungi. It is there- 
fore not surprising that lichens are a favourite habitat for parasitic fungi. 
Stahl 1 has noted that the lichens themselves flourish best where there is 
frequent moistening by rain or dew with equally frequent drying which 
effectively prevents the growth of fungi. Species of Peltigera are however 
able to live in damp conditions : without being injured, they have been 
observed to maintain their vigour when cultivated in a very moist hot- 
house while all the other forms experimented with were attacked and finally 
destroyed by various fungi. 

Lindsay 2 devoted a great deal of attention to the microscopic study of 
the minute fruiting bodies so frequently present on lichen thalli and published 
descriptions of microlichens, microfungi and spermogonia. He and others 
naturally considered these parasitic organisms to be in many cases either 
the spermogonia or pycnidia of the lichen itself. It is often not easy to 
determine their relationship or their exact systematic position ; many of 
them are still doubtful forms. 

There exists however a very large number of fully recognized parasitic 
microfungi belonging to various genera. Lindsay discovered many of them. 
Zopf 3 has given exact descriptions of a series of forms, with special reference 
to their effect on the host thallus. In an early paper he described a species, 
Pleospora collematum, that he found on Physma compactum and other Colle- 
maceae. The hyphae of the parasite differed from those of the host in being 
of a yellow colour; they did not penetrate or spread far, being restricted to 
rhizoid-like filaments at the base of their fruiting bodies (perithecia and 
pycnidia). Their presence caused a slight protuberance but otherwise did 
no harm to the host ; the Nostoc cells in their immediate vicinity were even 
more brightly coloured than in other parts of the thallus. In another paper 4 
he gives an instance of gall-formation in Collema pulposum induced by the 
presence of the fungus Didymosphaeria pulposi. Small protuberances were 
formed on the margins of the apothecia, more rarely on the lobes of the 
thallus, each one the seat of a perithecium of the fungus. No damage was 
done to either constituent of the thallus. 

Agyrium flavescens grows parasitically on the under surface of Peltigera 
polydactyla. M. and Mme Moreau 5 found that the hyphae of the fungus 
spread between the medullary filaments of the lichen; no haustoria were 
1 Stahl 1904. 2 Lindsay 1859, 1869, 1871. 3 Zopf 1896. 4 Zopf 1898. 6 Moreau I9i6 3 . 



PARASITISM 267 

observed. The mature fruiting body had no distinct excipulum, but was 
surrounded by*a layer of dead lichen cells. 

It is not easy to determine the difference between parasites that are of 
fungal nature and those that are lichenoid ; but as a general rule the fungi 
may be recognized by their more transient character, very frequently by 
their effect on the host thallus, which is more harmful than that produced 
by lichens, and generally by their affinity to fungi rather than to lichens. 
Opinions differ and will continue to differ on this very difficult question. 

The number of such fungi determined and classified has gradually 
increased, and now extends to a very long list. Even as far back as 1896 
Zopf reckoned up 800 instances of parasitism of 400 species of fungi on 
about 350 different lichens and many more have been added. Abbe Vouaux 1 
is the latest writer on the subject, but his work is mostly a compilation of 
species already known. He finds representatives of these parasites in nine 
families of Pyrenomycetes and six of Discomycetes. He leaves out of account 
the much debated Coniocarps, but he includes with fungi all those that have 
been proved to be parasymbiotic, such as Abrotliallus. 

A number of fungus genera, such as Conida, etc., are parasitic only on 
lichens. Most of them have one host only; others, such as Tichothecium 
pygmaeum, live on a number of different thalli. Crustaceous species are often 
selected by the parasites, and no great damage, if any, is caused to these 
hosts, except when the fungus is seated on the disc of the apothecium, so 
that the spore-bearing capacity is lessened or destroyed. 

In some of the larger lichens, however, harmful effects are more visible. 
In Lobaria pulmonaria, the fruits of which are attacked by the Discomycete, 
Celidium Stictarum-, there is at first induced an increased and unusual forma- 
tion of lichen apothecia. These apothecia are normally seated for the most 
part on the margins of the lobes or pustules, but when they are invaded by 
the fungus, they appear also in the hollows between the pustules and even 
on the under surface of the thallus. In the large majority of cases the 
fungus is partly or entirely embedded in the thallus; the gonidia in the 
vicinity may remain green and healthy, or all the tissues in the immediate 
neighbourhood of the parasite may be killed. 

/. MYCETOZOA PARASITIC ON LICHENS. Mycetozoa live mostly on 
decayed wood, leaves, humus, etc. One minute species, L isterella paradoxa, 
always inhabits the podetia of Cladonia rangiferina. Another species, 
Hymenobolina parasitica, was first detected and described by Zukal 3 as a 
true parasite on the thallus of Physciaceae; it has since been recorded in the 
British Islands on Parmeliae*. This peculiar organism differs from other 
mycetozoa in that the spores on germination produce amoebae. These unite 
to form a rose-red plasmodium which slowly burrows into the lichen thallus 
1 Vouaux 1912, etc. 2 Bitter 1904. 3 Zukal 1893. 4 Lister 1911. 



268 BIONOMICS 

and feeds on the living hyphae. It is a minute species, but when abundant 
the plasmodia can just be detected with the naked eye as rosy specks 
scattered over the surface of the lichen. Later the grey sporangia are 
produced on the same areas. 

F. DISEASES OF LICHENS 

a. CAUSED BY PARASITISM. Zopf l has stated that of all plants, lichens 
are the most subject to disease, reckoning as diseases all the instances of 
parasitism by fungi or by other lichens. There are however only rare 
instances in which total destruction or indeed any permanent harm to the 
host is the result of such parasitism. At worst the trouble is localized and 
does not affect the organism as a whole. Some of these cases have been 
already noted under antagonistic symbiosis or parasymbiosis. Several 
instances have however been recorded where real injury has been caused 
by the penetration of some undetermined fungus mycelium. Zukal 2 records 
two such observed by him in Parmelia encansta and Physcia villosa : the 
thallus of the former was dwarfed and deformed by the presence of the alien 
mycelium, the latter was excited to abnormal proliferation. 

b. CAUSED BY CROWDING. Lichens suffer frequently from being over- 
grown by other lichens ; they may also be crowded out by other plants. 
My attention was called by Mr P. Thompson to a burnt plot of ground in 
Epping Forest, which, after the fire, had been colonized by Peltigera spuria. 
In the course of a few years, other vegetation had followed, depriving the 
lichen of space and light and gradually driving it out. When last examined 
only a few miserable specimens remained, and these were reduced in vitality 
by an attack of the lichen parasite Illosporium carneum. 

c. CAUSED BY ADVERSE CONDITIONS. Zukal considers as pathological, 
at least in origin, the cracking of the thallus so frequent in crustaceous 
lichens as well as in the more highly developed forms. As the cracks are 
beneficial in the aeration of the plant, they can hardly be regarded as 
symptoms of a diseased condition. The more evident ringed breaks in the 
cortex of Usneae, due probably to wind action, have more reason to be so 
regarded ; they are most pronounced in Usnea articulata, where the portions 
bounded by the rings are contracted and swollen, and a hollow space is 
formed between the cortex and the central axis. The swellings that are 
produced n lichen thalli, such as those of Umbilicaria and some species of 
Gyrophora, due to intercalary growth are normal to the plant, though occasion- 
ally the swollen weaker portions may become ruptured and the cortex be 
thrown off. As pathological also must be regarded the loss of cortex some- 
times occasioned by excessive soredial formation at the margins of the lobes: 

1 Zopf 1897. 2 Zukal X g 96) p< 258 . 



DISEASES OF LICHENS 269 

the upper cortex may be rolled back and eventually torn away; the gonidial 
layer is exposed and transformed into soredia which are swept away by the 
wind and rain, till finally only traces of the lower cortex are left. 

Zukal 1 has instanced, as a case of diseased condition observed by him, 
the undue thickening of the cortex in Pertusaria communis whereby the 
formation of the fruiting bodies is inhibited and even vegetative development 
is rendered impossible. There arrives finally a stage when splitting takes 
place and the whole thallus breaks down and disappears. As a rule however 
there need be no limit to the age of the lichen plant. There is no vital 
point or area in the thallus ; injury of one part leaves the rest unhurt, and 
any fragment in growing condition, if it combines both symbionts, can carry 
on the life of the plant, the constant renewal of gonidia preventing either 
decay or death. Barring accidents many lichens might exist as long as the 
world endures. 

G. HARMFUL EFFECT OF LICHENS 

One lichen only, Strigula complanata, a tropical species, has been proved 
to be truly and constantly parasitic. It grows'on the surface of thick leathery 
leaves such as those of Camellia-, etc. and the alga and fungus both penetrate 
the epidermis and burrow beneath the cuticle and outer cells, causing them 
to become brown. It undoubtedly injures the leaves. 

Friedrich 3 has given an isolated instance of the hold-fast hyphae of Usnea 
piercing through the cortex to the living tissue of the host, and not only 
destroying the middle lamella by absorption, but entering the cells. The 
Usnea plant was characterized by exceptionally vigorous growth. Practically 
all corticolous lichens are epiphytic and the injury they cause is of an acci- 
dental nature Crustaceous species on the outer bark occupy the dead 
cortical layers and seem to be entirely harmless 4 . The larger foliose and 
fruticose forms are not so innocuous: by their abundant enveloping growth 
they hinder the entrance of air and moisture, and thus impede the life of 
the higher plant. Gleditsch 5 , one of the earliest writers on Forestry, first 
indicated the possibly harmful effect of lichens especially on young trees 
and " in addition," he says, " they serve as cover for large numbers of small 
insects which are hurtful in many ways to the trees." Lindau 6 pointed out 
the damage done to pine-needles by Xantkoria parietina which grew round 
them like a cuff and probably choked the stomata, the leaves so clothed being 
mostly withered. Dufrenoy 7 states that he found the hyphae of a Parmelia 
entering a pine-needle by the stomata, and that the starch disappeared from 
the neighbouring parenchyma the cells of which tended to disintegrate. 

It is no uncommon sight to see neglected fruit trees with their branches 
crowded with various lichens, Evernia prunaslri, Ramalina farinacea, etc. 
Such lichens often find the lenticels a convenient opening for their hold-fasts 

1 Zukal 1896, p. 255. ' 2 Cunningham 1879. 3 F"edrich 1906, p. 401. * See p. 78. 

5 Gleditsch 1775, p. 31. 6 Lindau 1895, p. 53. 7 Dufrenoy 1881. 



2 7 o BIONOMICS 

and excercise a smothering effect on the trees. Lilian Porter 1 distinctly 
states that Ramalinae by their penetrating bases damage the tissues of the 
trees. The presence of lichens is however generally due to unhealthy con- 
ditions already at work. Friedrich 2 reported of a forest which he examined, 
in which the atmospheric moisture was very high, with the soil water 
scarce, that those trees that were best supplied with soil water were free 
from lichens, while those with little water at the base bore dead branches 
which gave foothold to a rich growth of the epiphytes. 

Experiments to free fruit trees from their coating of lichens were made 
by Waite 3 . With a whitewash brush he painted over the infested branches 
with solutions of Bordeaux mixture of varying strength, and found that this 
solution, commonly in use as a fungicide, was entirely successful. The trees 
were washed down about the middle of March, and some three weeks later 
the lichens were all dead, the fruticose and foliose forms had changed in 
colour to a yellowish or brownish tint and wer.e drooping and shrivelled. 

Waite was of opinion that the lichens did considerable damage to the 
trees, but it has been held by others that in very cold climates they may 
provide protection against severe frost. Instances of damage are however 
asserted by Bouly de Lesdain 4 . The bark of willows he found was a favourite 
habitat of numerous lichens: certain species, such as Xanthoria parietina, 
completely surrounded the branches, closing the stomata; others, such as 
Physcia ascendens, by the mechanical strain of the rhizoids, first wet and then 
dry, gradually loosened the outer bark and gave entry to fungi which com- 
pleted the work of destruction. 

H. GALL-FORMATION 

Several instances of gall-formation to a limited extent have been already 
noted as caused by parasitic fungi or lichens. Greater abnormality of develop- 
ment is induced in a few species by the presence of minute animals, mites, 
wood-lice, etc. Zopf 5 noted these deformations of the thallus in specimens 
of Ramalina Kullensis collected on the coasts of Sweden. The fronds were 
frequently swollen in a sausage-like manner, and branching was hindered or 
altogether prevented; apothecia were rarely formed, though pycnidia were 
abundant. Here and there, on the swollen portions of the thallus, small 
holes could be detected and other larger openings of elliptical outline, about 
\-\\ mm. in diameter, the margins of which had a nibbled appearance. 
Three types of small articulated animals were found within the openings: 
species of mites, spiders and wood-lice. Mites were the most constant and 
were more or less abundant in all the deformations; frequently a minute 
Diplopodon belonging to the genus Polyxenus was also met with. 

Zopf came to the conclusion that the gall-formation was mainly due to 
the mites: they eat out the medulla and possibly through some chemical 

1 Porter 1917. 2 Friedrich 1906. Waite 1893. 4 Lesdain 1912. 5 Zopf 1907. 



GALL-FORMATION 271 

irritation excite the algal zone and cortex to more active growth, so that an 
extensive tangential development takesplace. The small spiders mayexercise 
the same power; evidently the larger holes were formed by them. 

Later Zopf added to gall-deformed plants Ramalina scopnlorum van in- 
crassata and R. cuspidata var. crassa. He found in the- hollow swollen fronds 
abundant evidence of mites, but whether identical with those that attacked 
R. Kullensis could not be determined. These two Ramalinae are maritime 
species ; they are morphologically identical, as are also the deformed varieties, 
and the presence of mites, excreta, etc., are plainly visible in our British 
specimens. 

Bouly de Lesdain 1 found evidence of mite action in Ramalina far inacea 
collected from Pinus sylvestris on the dunes near Dunkirk. The cortex 
had been eaten off either by mites or by a small mollusc (Pupa muscorum] 
and the fronds had collapsed to a more or less convex compact mass. 
Somewhat similar deformations, though less pronounced, were observed in 
other Ramalinae. 

In Cladonia sylvatica and also in Cl. rangiformis Lesdain has indicated 
ff. abortiva Harm, as evidently the result of insect attack. In both cases the 
tips of the podetia are swollen, brown, bent and shrivelled. 

One of the most curious and constant effects, also worked out by Lesdain, 
occurs in Physcia hispida (Ph. stellaris var. tenella). In that lichen the 
gonidia at the tips of the fronds are scooped out and eaten by mites, so 
that the upper cortex becomes separated from the lower part of the thallus. 
As the hyphae of the cortex continue to develop, an arched hood is formed 
of a whitish shell-like appearance and powdery inside. Sometimes the 
mites penetrate at one point only, at other times the attack is at several 
places which may ultimately coalesce into one large cavity. In a crustaceous 
species, Caloplaca (Placodium) citrina he found constant evidence of the 
disturbing effect of the small creatures, which by their action caused the 
areolae of the thallus to grow into minute adherent squamules. A patho- 
logical variety, which he calls var. sorediosa, is distinguished by the presence 
of cup-like hollows which are scooped out by Acarinae and are filled by 
yellowish soredia. In another form, var. maritima, the margins of the areolae, 
occasionally the whole surface, become powdery with a citrine yellow 
efflorescence as a result of their nibbling. 

Zukal 2 adds to the deformations due to organic agents, the hypertrophies 
and abnormalities caused by climatic conditions. He finds such irregularities 
of structure more especially developed in countries with a very limited rain- 
fall, as in certain districts of Chili, Australia and Africa, where changes in 
cortex and rhizoids and proliferations of the thallus testify to the disturbance 
of normal development. 

1 Lesdain 1910. 2 Zukal 1896, p. 258. 



CHAPTER VII 

PHYLOGENY 

I. GENERAL STATEMENT 
A. ORIGIN OF LICHENS 

THOUGH lichens are very old members of the vegetable kingdom, as 
symbiotic plants they yet date necessarily from a time subsequent to the 
evolution of their component symbionts. Phylogeny of lichens begins with 
symbiosis. 

The algae, which belong to those families of Chlorophyceae and Myxo- 
phyceae that live on dry land, had become aerial before their association 
with fungi to form lichens. They must have been as fully developed then 
as now, since it is possible to refer them to the genus or sometimes even to 
the species of free-living forms. The fungus hyphae have combined with a 
considerable number of different algae, so that, even as regards the algal 
symbiont, lichens are truly polyphyletic in origin. 

The fungus is, however, the dominant partner, and the principal line of 
development must be traced through it, as it provides the reproductive organs 
of the plant. Representatives of two great groups of fungi are associated 
with lichens: Basidiomycetes, found in only a few genera, and Ascomycetes 
which form with the various algae the great bulk of lichen families. In 
respect of their fungal constituents lichens are also polyphyletic, and more 
especially in the Ascolichens which can be traced back to several starting 
points. But though lichens have no common origin, the manner of life is 
common to them all and has influenced them all in certain directions: they 
are fitted for a much longer existence than that of the fungi from which they 
started; and both the thallus and the fruiting bodies at least in the sub- 
class Ascolichens can persist through great climatic changes, and can pass 
unharmed through prolonged periods of latent or suspended vitality. 

Another striking note of similarity that runs through the members of this 
sub-class, with perhaps the exception of the gelatinous lichens, is the formation 
of lichen-acids which are excreted by the fungus. These substances are 
peculiar to lichens and go far to mark their autonomy. The production of 
the acids and the many changes evolved in the vegetative thallus suggest the 
great antiquity of lichens. 



ORIGIN OF LICHENS 273 

B. ALGAL ANCESTORS 

It is unnecessary to look far for the algae as they have persisted through 
the ages in the same form both without and within the lichen thallus. By 
many early lichenologists the free-living algae, similar in type to lichen algae, 
were even supposed to be lichen gonidia in a depauperate condition and 
were, for that reason, termed by Wallroth " unfortunate brood-cells." In the 
condition of symbiosis they may be considerably modified, but they revert 
to their normal form, and resume their normal life-history of spore production, 
etc., under suitable and free culture. The different algae taking part in 
lichen-formation have been treated in an earlier chapter 1 . 

C. FUNGAL ANCESTORS 

a. HVMENOLICHENS. The problem of the fungal origin in this sub-class 
is comparatively simple. It contains but three genera of tropical lichens which 
are all associated with Myxophyceae, and the fungus in them, to judge from 
the form and habit of the plants, is a member of the Thelephoraceae. It 
may be that Hymenolichens are of comparatively recent origin and that the 
fungi belonging to the Basidiomycetes had, in the course of time, become 
less labile and less capable of originating a new method of existence. What- 
ever the reason, they lag immeasurably behind Ascomycetes in the formation 
of lichens. 

b. ASCOLICHENS. Lichens are again polyphyletic within this sub-class. 
The main groups from which they are derived are evident. Whether there 
has been a series of origins within the different groups or a development 
from one starting point in each it would be difficult to determine. In any 
case great changes have taken place after symbiosis became established. 

The main divisions within the Ascolichens are related to fungi thus: 

Series I. Pyrenocarpineae I 

. \ to Pyrenomycetes. 

2. Comocarpmeae ) 

3. Graphidineae to Hysteriaceae. 

4. Cyclocarpineae to Discomycetes. 

II. THE REPRODUCTIVE ORGANS 

A. THEORIES OF DESCENT IN ASCOLICHENS 

It has been suggested that ascomycetous fungi, from which Ascolichens 
are directly derived, are allied to the Florideae, owing to the appearance of 
a trichogyne in the carpogonium of both groups. That organ in the red sea- 
weeds is a long delicate cell in direct communication with the egg-cell of 
the carpogonium. It is a structure adapted to totally submerged conditions, 
and fitted to attach the floating spermatia. 

1 See p. 51. 
S.L. 18 



274 PHYLOGENY 

In fungi there is also a structure considered as a trichogyne 1 , which, in 
the Laboulbeniales, is a free, simple or branching organ. There is no other 
instance of any similar emergent cell or cells connected with the ascogonium 
of the Ascomycetes, though the term has been applied in these fungi to 
certain short hyphal branches from the ascogonium which remain embedded 
in the tissue. In the Ascomycetes examined all traces of emergent receptive 
organs, if they ever existed, have now disappeared ; in some few there are 
ipossible internal survivals which never reach the surface. 

In Ascolichens, on the contrary, the "trichogyne," a septate hyphal 
branch extending upwards from the ascogonium, and generally reaching the 
open, has been demonstrated in all the different groups except, as yet,- in 
the Coniocarpineae which have not been investigated. Its presence is a 
strong point in the argument of those who believe in the Floridean ancestry 
of the Ascomycetes. It should be clearly borne in mind that Ascolichens 
are evolved from the Ascomycetes: these latter stand between them and 
any more remote ancestry. 

In the Ascomycetes, there is a recognized progression of development 
in the form of the sporophore from the closed perithecium of the Pyreno- 
mycetes and possibly through the vHysteriaceae, which are partially closed, 
to the open ascocarp of the Discomycetes. If the fungal and lichenoid 
" trichogyne " is homologous with the carpogonial organ in the Florideae, 
then it must have been retained in all the groups of Ascomycetes as an 
emergent structure, and as such passed on from them to their lichen 
derivatives. Has that organ then disappeared from fungi since symbiosis 
began ? There is no trace of it now, except as already stated in Laboul- 
beniales with which lichens are unconnected. 

Were Ascolichens monophyletic in origin, one could more easily suppose 
that both the fungal and lichen series might have started at some early stage 
from a common fungal ancestor possessing a well-developed trichogyne 
which has persisted in lichens, but has been reduced to insignificance in 
fungi, while fruit development proceeded on parallel lines in both. There is 
no evidence that such progression has taken place among lichens ; the theory 
of a polyphyletic origin for the different series seems to be unassailable. At 
the same time, there is no evidence to show in which series symbiosis started 
first. 

It is more reasonable to accept the polyphyletic origin, as outlined above, 
from forms that had already lost the trichogyne, if they ever really possessed 
it, and to regard the lichen trichogyne as a new organ developing in lichens 
in response to some requirement of the deep-seated ascogonium. Its sexual 
function still awaits satisfactory proof, and it is wiser to withhold judgment 
as to the service it renders to the developing fruit. 

1 See p. \li et seq. 



REPRODUCTIVE ORGANS 275 

B. RELATION OF LICHENS TO FUNGI 

a. PYRENOCARPINEAE. In Phycolichens (containing blue-green gonidia) 
and especially in the gelatinous forms, fructification is nearly always a more 
or less open apothecium. The general absence of the perithecial type is 
doubtless due to the gelatinous consistency of the vegetative structure; it is 
by the aid of moisture that the hymenial elements become turgid enough 
to secure the ejection of the spores through the narrow ostiole of the peri- 
thecium, and this process would be frustrated were the surrounding and 
enveloping thallus also gelatinous. There is only one minutely foliose or 
fruticose gelatinous family, the Pyrenidiaceae, in which Pyrenomycetes are 
established, and the gonidia, even though blue-green, have lost the gelatinous 
sheath and do not swell up. 

In Archilichens (with bright-green gonidia), perithecial fruits occur 
frequently ; they are nearly always simple and solitary; in only a few families 
with a few representatives, is there any approach to the stroma formation so 
marked among fungi. The single perithecium is generally semi-immersed 
in the thallus. It may be completely surrounded by a hyphal " entire " wall, 
either soft and waxy or dark coloured and somewhat carbonaceous. In 
numerous species the outer protective wall covers only the upper portion 
that projects beyond the thallus, and such a perithecium is described as 
" dimidiate," a type of fruit occurring in several genera, though rare among 
fungi. 

As to internal structure, there is a dissolution and disappearance of the 
paraphyses in some genera, their protective function not being so necessary 
in closed fruits, a character paralleled in fungi. There is a great variety of 
spore changes, from being minute, simple and colourless, to varied septation, 
general increase in size, and brown colouration. The different types may 
be traced to fungal ancestors with somewhat similar spores, but more 
generally they have developed within the lichen series. From the life of the 
individual it is possible to follow the course of evolution, and the spores of 
all species begin as simple, colourless bodies; in some genera they remain 
so, in others they undergo more or less change before reaching the final 
stage of colour or septation that marks the mature condition. 

As regards direct fungal ancestors, the Pyrenocarpineae, with solitary 
perithecia, are nearest in fruit structure to the Mycosphaerellaceae, in which 
family are included several fungus genera that are parasitic on lichens such 
as Ticothecium, Mullerella, etc. In that family occurs also the genus Stigmatea, 
in which the perithecia in form and structure are very similar to dimidiate 
Vernicariae. 

Zahlbruckner 1 has suggested as the starting point for the Verrucariaceae 

1 Zahlbruckner 1903. 

18 2 



276 PHYLOGENY 

the fungus genus Verrncula. It was established by Steiner 1 to include two 
species, V. cahirensis and V. aegyptica, their perithecia being exactly similar 
to those of Verrucaria? in which genus they were originally placed. Both 
are parasitic on species of Caloplaca (P lacodium}. The former, on C. gilvella, 
transforms the host thallus to the appearance of a minutely lobed Placodium ; 
the latter occupies an island-like area in the centre of the thallus of Caloplaca 
interveniens, and gives it, with its accompanying parasite, the character of 
an Endopyrenium (Dermatocarpon), while the rest of the thallus is normal 
and fertile. 

Zahlbruckner may have argued rightly, but it is also possible to regard 
these rare desert species as reversions from an originally symbiotic to a purely 
parasitic condition. Reinke came to the conclusion that if a parasitic 
species were derived directly from a lichen type, then it must still rank as 
a lichen, a view that has a direct bearing on the question. The parallel 
family of Pyrenulaceae which have Trentepohlia gonidia is considered by 
Zahlbruckner to have originated from the fungus genus Didymella. 

Compound or stromatoid fructifications occur once and again in lichen 
families; but, according to Wainio 3 , there is no true stroma formation, only 
a pseudostroma resulting from adhesions and agglomerations of the thalline 
envelopes or from cohesions of the margins of developing fruit bodies. 
These pseudostromata are present in the genera Chiodecton and Glyphis 
(Graphidineae) and in Trypethelium, Mycoporium, etc. (Pyrenocarpineae). 
This view of the nature of the compound fruits is strengthened, as Wainio 
points out, by the presence in certain species of single apothecia or perithecia 
on the same specimen as the stromatoid fruits. 

b. CONIOCARPINEAE. This subseries is entirely isolated. Its peculiarity 
lies in the character of the mature fruit in which the spores, owing to the 
early breaking down of the asci, lie as a loose mass in the hymenium, while 
dispersal is delayed for an indefinite time. This type of fruit, termed a 
mazaedium by Acharius, is in the form of a stalked or sessile roundish head 
the capitulum closed at first and only half-open at maturity rarely, as in 
Cyphelium, an exposed disc. There is a suggestion, but only a suggestion, of a 
similar fructification in the tropical fungus Camillea in which there is some- 
times a stalk with one or more perithecia at the tip, and in some species early 
disintegration of the asci, leaving spore masses 4 . But neither in fungi nor in 
other lichens is there any obvious connection with Coniocarpineae. In some 
of the genera the fungus alone forms the stalk and the wall of the capitulum ; 
in others the thallus shares in the fruit-formation growing around it as an 
amphithecium. 

The semi-closed fruits point to their affinity with Pyrenolichens, though 

1 Steiner 1896. 2 Muller-Argau 1880. 3 Wainio 1890, p. xxiii. 4 Lloyd 1917. 



REPRODUCTIVE ORGANS 277 

they are more advanced than these judging from the thalline wall that is 
present in some genera and also from the half-open disc at maturity. The 
latter feature has influenced some systematists to classify the whole subseries 
among Cyclocarpineae. The thallus, as in Sphaerophorus, reaches a high 
degree of fruticose development ; in other genera it is crustaceous without 
any formation of cortex, while in several genera or species it is non-existent, 
the fruits being parasites on the thalli of other lichens or saprophytes on 
dead wood, humus, etc. These latter both parasites and saprophytes 
are included by Rehm 1 and others among fungi, which has involved the 
breaking up of this very distinctive series. Rehm has thus published as 
Discomycetes the lichen genera Sphinctrina, Cyp helium, Coniocybe, Ascoliunit 
Calicium and Stenocybe, since some or all of their species are regarded by 
him as fungi. 

Reinke 2 in his lichen studies states that it might not be impossible for 
a saprophytic fungus to be derived from a crustaceous lichen a case of 
reversion but that no such instance was then known. More exact studies 3 
of parasymbiosis and antagonistic symbiosis have shown the wide range of 
possible life-conditions, and such a reversion does not seem improbable. We 
must also bear in mind that in suitable cultures, lichen hyphae can be grown 
without gonidia: they develop in that case as saprophytes. 

On Reinke's 2 view, however, that these saprophytic species, belonging to 
different genera in the Coniocarpineae, are true fungi, they would represent 
the direct and closely related ancestors of the corresponding lichen genera, 
giving a polyphyletic origin within this group. As fungus genera he has 
united them in Protocaliciaceae, and the representatives among fungi he 
distinguishes, as does Wainio 4 , under such names as Mycocalicium and 
Mycocon iocybe. 

If we might consider the saprophytic forms as also retrogressive lichens, 
a monophyletic origin from some remote fungal ancestor would prove a more 
satisfactory solution of the inheritance problem. This view is even supported 
by a comparison Reinke himself has drawn between the development of the 
fructification in Mycocalicium parietinum, a saprophyte, and in his view a 
fungus, and Chaenotheca cJirysocephala, a closely allied lichen. Both grow on 
old timber. In the former (the fungus), the mycelium pervades the outer 
weathered wood-cells, and the fruit stalk rises from a clump of brownish 
hyphae; there is no trace of gonidia. ChaenotJieca chrysocephala differs in the 
presence of gonidia which are associated with the mycelium in scattered 
granular warts; but the fruit stalk here also rises directly from the mycelium 
between the granules. The presence of a lichen thallus chiefly differentiates 
between the two plants, and this thallus is not a casual or recent association; 
it is constant and of great antiquity as it is richly provided with lichen-acids. 

1 Rehm 1890. 2 Reinke 1894. 3 See p. 260. 4 Wainio 1890. 



2 ;8 PHYLOGENY 

Reinke has indicated the course of evolution within the series but that 
is on the lines of thalline development and will be considered later. 

c. GRAPHIDINEAE. This series contains a considerable variety of lichen 
forms, but all possess to a more or less marked degree the linear form of 
fructification termed a "lirella" which has only a slit-like opening. There 
is a tendency to round discoid fruits in the Roccellae and also in the Arthoniae; 
the apothecia of the latter, called by early lichenologists "ardellae," are with- 
out margins. In nearly all there is a formation of carbonaceous black tissue 
either in the hypothecium or in the proper margins. In some of them the 
paraphyses are branched and dark at the tips, the branches interlocking to 
form a strong protective epithecium. There are, however, constant exceptions, 
in some particular, to any generalization in genera and in species. Miiller- 
Argau's 1 pronouncement might be held to have special reference to Graphi- 
dineae: "that in any genus, species or groups of species are to be found 
which outwardlyshew something that is peculiar, thoughof slightimportance." 
The most constant type of gonidium is Trentepbhlia, but Palmella and 
Phycopeltis occasionally occur. The spores are various in colour and form ; 
they are rarely simple. 

The genus Arthonia is derived from a member of the Patellariaceae, from 
which family many of the Discomycetes have arisen. The course of develop- 
ment does not follow from a closed to an open fruit ; the apothecium is open 
from the first, and growth proceeds from the centre outwards, the fertile cells 
gradually pushing aside the sterile tissue of the exterior. The affinity of 
Xylographa (with Palmella gonidia) is to be found in Stictis in the fungal 
family Stictidaceae, the apothecia of Stictis being at first closed, then open, 
and with a thick margin ; Xylographa has a more elongate lirella fruit, though 
otherwise very similar, and has a very reduced thallus. Rehm 2 has classified 
Xylographa as a fungus. 

The genera with linear apothecia are closely connected with Hysteriaceae, 
and evidently inherit their fruit form severally from that family. There is 
thus ample evidence of polyphyletic descent in the series. Stromatoid fruits 
occur in Chiodectonaceae, with deeply sunk, almost closed disc, but they 
have evidently evolved within the series, possibly from a dividing up of the 
lirellae. 

In Graphidineae there are also forms, more especially in Arthoniaceae, 
on the border line between lichens and fungi: those with gonidia being 
classified as lichens, those without gonidia having been placed in corre- 
sponding genera of fungi. These latter athalline species live as parasites or 
saprophytes. 

The larger number of genera have a poorly developed thallus; in many 
of them it is embedded within the outer periderm-cells of trees, and is known 

1 Muller-Argau 1862. 2 Rehm 1890. 



REPRODUCTIVE ORGANS 279 

as " hypophloeodal." But in some families, such as Roccellaceae, the thallus 
attains a very advanced form and a very high production of acids. 

The conception of Graphidineae as a whole is puzzling, but one or other 
characteristic has brought the various members within the series. It is in 
this respect an epitome of the lichen class of which the different groups, 
with all their various origins and affinities, yet form a distinct and well-defined 
section of the vegetable kingdom. 

d. CYCLOCARPINEAE. This is by far the largest series of lichens. The 
genera are associated with algae belonging both to the Myxophyceae and 
the Chlorophyceae, and from the many different combinations are produced 
great variations in the form of the vegetative body. The fruit is an emergent, 
round or roundish disc or open apothecium in all the members of the series 
except Pertusariaceae, where it is partially immersed in thalline " warts." 
In its most primitive form, described as "biatorine" or "lecideine," it may 
be soft and waxy {Biatorci) or hard and carbonaceous (Lecidea), in the latter 
the paraphyses being mostly coloured at the tips ; these are either simple or 
but sparingly branched, so that the epithecium is a comparatively slight 
structure. The outer sterile tissue forms a protective wall or "proper margin" 
which may be entirely pushed aside, but generally persists as a distinct rim 
round the disc. 

A great advance within the series arose when the gonidial elements of 
the thallus took part in fruit-formation. In that case not only is the 
hymenium generally subtended by a layer of algae, but thalline tissue con- 
taining algae grows up around the fruit, and forms a second wall or thalline 
margin. This type of apothecium, termed " lecanorine," is thus intimately 
associated with the assimilating tissue and food supply, and it gains in 
capacity of ascus renewal and of long duration. This development from 
non-marginate to marginate ascomata is necessarily an accompaniment of 
symbiosis. 

There is no doubt that the Cyclocarpineae derive from some simple 
form or forms of Discomycete in the Patellariaceae. The relationship 
between that family and the lower Lecideae is very close. Rehm 1 finds the 
direct ancestors of Lecidea itself in the fungus genus, Patinella, in which the 
apothecia are truly lecideine in character open, flat and slightly margined, 
the hypothecium nearly always dark-coloured and the paraphyses branched, 
septate, clavate and coloured at the tips, forming a dark epithecium. More 
definitely still he describes Patinella atroviridis, a new species he discovered, 
as in all respects a Lecidea, but without gonidia. 

In the crustaceous Lecideaceae, a number of genera have been delimited 
on spore characters colourless or brown, and simple or variously septate. 
In Patellariaceae as described by Rehm are included a number of fungus 

1 Rehm 1890. 



2 8o PHYLOGENY 

genera which correspond to these lichen genera. Only two of them 
Patinella and Patellaria are saprophytic ; in all the other genera of the 
family, the species with very few exceptions are parasitic on lichens : they 
are parasymbionts sharing the algal food supply ; in any case, they thrive 
on a symbiotic thallus. 

Rehm unhesitatingly derives the corresponding lichen genera from these 
fungi. He takes no account of the difficulty that if these parasitic (or sapro- 
phytic) fungi are primitive, they have yet appeared either later in time than 
the lichens on which they exist, or else in the course of ages they have 
entirely changed their substratum. 

He has traced, for instance, the lichen, Buellia, to a saprophytic fungus 
species, Karschia lignyota, to a genus therefore in which most of the species 
are parasitic on lichens and have generally been classified as parasitic lichens. 
There is no advance in apothecial characters from the fungus, Karschia, to 
Buellia, merely the change to symbiosis. It therefore seems more in accord- 
ance with facts to regard Buellia as a genus evolved within the lichen series 
from Patinella through Lecidea, and to accept these species of Karschia on 
the border line as parasitic, or even as saprophytic, reversions from the 
lichen status. We may add that while these brown-spored lichens are fairly 
abundant, the corresponding athalline or fungus forms are comparatively 
few in number, which is exactly what might be expected from plants with 
a reversionary history. 

Occasionally in biatorine or lecideine species with a slight thalline 
development all traces of the thallus disappear after the fructification has 
reached maturity. The apothecia, if on wood or humus, appear to be 
saprophytic and would at first sight be classified as fungi. They have un- 
doubtedly retained the capacity to live at certain stages, or in certain con- 
ditions, as saprophytes. 

The thallus disappears also in some species of the crustaceous genera 
that possess apothecia with a thalline margin, and the fruits may be left 
stranded and solitary on the normal substratum, or on some neighbouring 
lichen thallus where they are more or less parasitic ; but as the thalline 
margin persists, there has been no question as to their nature and affinity. 

Rehm suggests that many species now included among lichens may be 
ultimately proved to be fungi ; but it is equally possible that the reverse may 
be the case, as for instance Bacidiaflavovirescens, held by Rehm and others to 
be a parasitic fungus species, but since proved by Tobler 1 to be a true lichen. 

A note by Lightfoot 2 , one of our old-time botanists who gave lichens a 
considerable place in his Flora, foreshadows the theory of evolution by 
gradual advance, and his views offer a suggestive commentary on the subject 
under discussion. He was debating the systematic position of the maritime 

1 Tobler 191 1 2 , p. 407. 2 Lightfoot 1777, p. 965. 



REPRODUCTIVE ORGANS 281 

lichen genus Lichina, considered then a kind of Fucus, and had observed 
its similarity with true lichens. " The cavity," he writes, " at the top of the 
fructification (in Lichind) is a proof how nearly this species of Fucus is 
related to the scutellated lichens. Nature disdains to be limited to the 
systematic rules of human invention. She never makes any sudden starts 
from one class or genus to another, but is regularly progressive in all her 
works, uniting the various links in the chain of beings by insensible con- 
nexions." 

III. THE THALLUS 

A. GENERAL OUTLINE OF DEVELOPMENT 

a. PRELIMINARY CONSIDERATIONS. The evolution of lichens, as such, 
has reference mainly to the thallus. Certain developments of the fructification 
are evident, but the changes in the reproductive organs have not kept pace 
with those of the vegetative structures: the highest type of fruit, for instance, 
the apothecium with a thalline margin, occurs in genera and species with a 
very primitive vegetative structure as well as in those that have attained 
higher development. 

Lichens are polyphyletic as regards their algal, as well as their fungal, 
ancestors, so that it is impossible to indicate a straight line of progression, 
but there is a general process of thalline development which appears once 
and again in the different phyla. That process, from simpler to more com- 
plicated forms, follows on two lines: on the one there is the endeavour to 
increase the assimilating surface, on the other the tendency to free the plant 
from the substratum. In both, the aim has been the same, to secure more 
favourable conditions for assimilation and aeration. Changes in structure 
have been already described 1 , and it is only needful to indicate here the main 
lines of evolution. 

b. COURSE OF EVOLUTION IN HYMENOLICHENS. There is but little 
trace of development in these lichens. The fungus has retained more or less 
the form of the ancestral Thelephora which has a wide-spreading superficial 
basidiosporous hymenium. Three genera have been recognized, the differences 
between them being due to the position within the thallus, and the form of 
the Scytonema that constitutes the gonidium. The highest stage of develop- 
ment and of outward form is reached in Cora, in which the gonidial zone 
is central in the tissue and is bounded above and below by strata of hyphae. 

c. COURSE OF EVOLUTION IN ASCOLICHENS. It is in the association 
with Ascomycetes that evolution and adaptation have had full scope. In 
that subclass there are four constantly recurring and well-marked stages 
of thalline development, (i) The earliest, most primitive stage, is the 

1 See Chap. III. 



282 PHYLOGENY 

crustaceous: at first an accretion of separate granules which may finally be 
united into a continuous crust with a protective covering of thick-walled 
amorphous hyphae forming a " decomposed " cortex. The extension of 
a granule by growth in one direction upwards and outwards gives detach- 
ment from the substratum, and originates (2) the squamule which is, how- 
ever, often of primitive structure and attached to the support, like the granule, 
by the medullary hyphae. Further growth of the squamule results in (3) 
the foliose thallus with all the adaptations of structure peculiar to that form. 
In all of these, the principal area of growth is round the free edges of the 
thallus. A greater change takes place in the advance to (4) the fruticose 
type in which the more active growing tissue is restricted to the apex, and 
in which the frond or filament adheres at one point only to the support, a 
new series of strengthening and other structures being evolved at the same 
time. 

The lichen fungi associate, as has been already stated, with two different 
types of algae: those combined with the Myxophyceae have been designated 
Phycolichenes, those with Chlorophyceae as Archilichenes. The latter pre- 
dominate, not only in the number of lichens, but also in the more varied 
advance of the thallus, although, in many instances, genera and species of 
both series may be closely related. 

B. COMPARATIVE ANTIQUITY OF ALGAL SYMBIONTS 

One of the first questions of inheritance concerns the comparative an- 
tiquity of the two gonidial series: with which kind of alga did the fungus 
first form the symbiotic relationship ? No assistance in solving the problem 
is afforded by the type of fructification. The fungus in Archilichens is 
frequently one of the more primitive Pyrenomycetes, though more often a 
Discomycete, while in Phycolichens Pyrenomycetes are very rare. There 
is, as already stated, no corelation of advance between the fruit and the 
thallus, as the most highly evolved apothecia with well-formed thalline 
margins are constantly combined with thalli of low type. 

Forssell 1 gave considerable attention to the question of antiquity in his 
study of gelatinous crustaceous lichens in the family Pyrenopsidaceae, termed 
by him Gloeolichens, and he came to the conclusion that Archilichens 
represented the older combination, Phycolichens being comparatively.young. 

His view is based on a study of the development of certain lichen fungi 
that seem able to adapt themselves to either kind of algal symbiont. He 
found 1 in Euopsis (Pyrenopsis) granatina, one of the Pyrenopsidaceae, that 
certain portions of the thallus contained blue-green algae, while others con- 
tained Palmella, and that these latter, though retrograde in development, 

1 Forssell 1885. 



THE THALLUS 283 

might become fertile. The granules with blue-green gonidia were stronger, 
more healthy and capable of displacing those with Palmella, but not of 
bearing apothecia, though spermogonia were embedded in them a first step, 
according to Forssell, towards the formation of apothecia. These granules, 
not having reached a fruiting stage, were reckoned to be of a more recent 
type than those associated with Palmella. In other instances, however, the 
line of evolution has been undoubtedly from blue-green to more highly 
evolved bright-green thalli. 

The striking case of similarity between Psoroma hypnorum (bright-green) 
and Pannaria rubiginosa (blue-green) may also be adduced. Forssell con- 
siders that Psoroma is the more ancient form, but as the fungus is adapted 
to associate with either kind of alga, the type of squamules forming the 
thallus may be gradually transformed by the substitution of blue-green for 
the earlier bright-green the Pannaria superseding the Psoroma. There is 
a close resemblance in the fructification that is of the fungus in these two 
different lichens. 

Hue 1 shares Forssell's opinion as to the greater antiquity of the bright- 
green gonidia and cites the case of Solorina crocea. In that lichen there is 
a layer of bright-green gonidia in the usual dorsiventral position, below the 
upper cortex. Below this zone there is a second formed entirely of blue- 
green cells. Hue proved by his study of development in Solorina that the 
bright-green were the normal gonidia of the thallus, and were the only ones 
present in the growing peripheral areas; the blue-green were a later addition, 
and appeared first in small groups at some distance from the edge of the 
lobes. 

The whole subject of cephalodia-development 2 has a bearing on this 
question. These bodies always contain blue-green algae, and are always 
associated with Archilichens. Mostly they occur as excrescences, as in 
Stereocaulon and in Peltigera. The fungus of the host-lichen though normally 
adapted to bright-green algae has the added capacity of forming later a sym- 
biosis with the blue-green. This tendency generally pervades a whole genus 
or family, the members of which, as in Peltigeraceae, are too closely related 
to allow as a rule of separate classification even when the algae are totally 
distinct. 

C. EVOLUTION OF PHYCOLICHENS 

The association of lichen-forming fungi with blue-green algae may have 
taken place later in time, or may have been less successful than with the 
bright-green: they are fewer in number, and the blue-green type of thallus 
is less highly evolved, though examples of very considerable development 
are to be found in such genera as Peltigera, Sticta or Nephromium. 
1 Hue 191 1 1 . 2 See p. 133. 



284 PHYLOGENY 

a. GLOEOLICHENS. Among crustaceous forms the thallus is generally 
elementary, more especially in the Gloeolichens (Pyrenopsidaceae). The 
algae of that family, Gloeocapsa, Xanthocapsa or Chroococcus, are furnished 
with broad gelatinous sheaths which, in the lichenoid state, are penetrated 
and traversed by the fungal filaments, a branch hypha generally touching 
with its tip the algal cell-wall. Under the influence of symbiosis, the algal 
masses become firmer and more compact, without much alteration in form; 
algae entirely free from hyphae are often intermingled with the others. Even 
among Gloeolichens there are signs of advancing development both in the 
internal structure and in outward form. Lobes free from the substratum, 
though very minute, appear in the genus Paulia, the single species of which 
comes from Polynesia. Much larger lobes are characteristic of Thyrea, a 
Mediterranean and American genus. The fruticose type, with upright fronds 
of minute size, also appears in our native genus Synalissa. It is still more 
marked in the coralloid thalli of Peccania and Phleopeccania. In most of 
these genera there is also a distinct tendency to differentiation of tissues, 
with the gonidia congregating towards the better lighted surfaces. The only 
cortex formation occurs in the crustaceous genus Forssellia in which, according 
to Zahlbruckner 1 , it is plectenchymatous above, the thallus being attached 
below by hyphae penetrating the substratum. In another genus, Anema?, 
which is minutely lobate-crustaceous, the internal hyphae form a cellular 
network in which the algae are immeshed. As regards algal symbionts, 
the members of this family are polyphyletic in origin. 

b. EPHEBACEAE AND COLLEMACEAE. In Ephebaceae the algae are 
tufted and filamentous, Scytonema, Stigonema or Rivularia, the trichomes of 
which are surrounded by a common gelatinous sheath. The hyphae travel 
in the sheath alongside the cell-rows, and the symbiotic plant retains the 
tufted form of the alga as in Lichina with Rivularia, Leptogidium with Scyto- 
nema, and Ephebe with Stigonema. The last named lichen forms a tangle of 
intricate branching filaments about an inch or more in length. The fruticose 
habit in these plants is an algal characteristic ; it has not been acquired as a 
result of symbiosis, and does not signify any advance in evolution. 

A plectenchymatous cortex marks some progress here also in Lepto- 
dendriscum, Leptogidium and Polychidium, all of which are associated with 
Scytonema. These genera may well be derived from an elementary form 
such as Thermutis. They differ from each other in spore characters, etc., 
Polychidium being the most highly developed with its cortex of two cell- 
rows and with two-celled spores. 

Nostoc forms the gonidium of Collemaceae. In its free state it is extremely 
gelatinous and transmits that character more or less to the lichen. In the 
crustaceous genus Physma, which forms the base of the Collema group or 

1 Zahlbruckner 1907. 2 Reinke 1895. 



THE THALLUS 285 

phylum, there is but little difference in form between the thalline warts of 
the lichen crust and the original small Nostoc colonies such as are to be 
found on damp mosses, etc. 

In Collema itself, the less advanced species are scarcely more than crusts, 
though the more developed show considerable diversity of lobes, either short 
and pulpy, or spreading out in a thin membrane. The Nostoc chains pervade 
the homoiomerous thallus, but in some species they lie more towards the 
upper surface. There is no cortex, though once and again plectenchyma 
appears in the apothecial margin, both in this genus and in Leprocolletna 
which is purely crustaceous. 

Leptogium is a higher type than Col/etna, the thallus being distinguished 
by its cellular cortex. The tips of the hyphae, lying close together at the 
surface, are cut off by one or more septa, giving a one- or several-celled 
cortical layer. The species though generally homoiomerous are of thinner 
texture and are less gelatinous than those of Collema. 

c. PVRENIDIACEAE. This small family of pyrenocarpous Phycolichens 
may be considered here though its affinity, through the form of the fruiting 
body, is with Archilichens. The gonidia are species of Nostoc, Scytoncma 
and Stigonema. There are only five genera; one of these, Eolichcn, contains 
three species, the others are monotypic. 

The crustaceous genera have a non-corticate thallus, but an advance to 
lobate form takes place in PlacotJielium, an African genus. The two genera 
that show most development are both British: Corisciiun (Normandina), 
which is lobate, heteromerous and corticate though always sterile and 
Pyreniciium which is fruticose in habit ; the latter is associated with Nostoc 
and forms a minute sward of upright fronds, corticate all round ; the peri- 
thecium is provided with an entire wall and is immersed in the thallus. 

If the thallus alone were under consideration these lichens would rank 
with Pannariaceae. 

d. HEPPIACEAE AND PANNARIACEAE. The next stage in the develop- 
ment of Phycolichens takes place through the algae, Scytonema and Nostoc, 
losing not only their gelatinous sheaths, but also, to a large extent, their 
characteristic forms. Chains of cells can frequently be observed, but accurate 
and certain identification of the algal genus is only possible by making 
separate cultures of the gonidia. 

Scytonema forms the gonidium of the squamulose Heppiaceae consisting 
of the single genus Heppia. The ground tissue of the species is either 
wholly of plectenchyma with algae in the interstices, or the centre is occupied 
by a narrow medulla of loose filaments. 

In the allied family Pannariaceae, a number of genera contain Scytonema 
or Nostoc, while two, Psoroma and Psoromaria, have bright-green gonidia. 



286 PHYLOGENY 

The thallus varies from crustaceous or minutely squamulose, to lobes of 
fair dimension in Parmeliella and in Hydrothyria venosa, an aquatic lichen. 
Plectenchyma appears in the upper cortex of both of these, and in the 
proper margin of the apothecia, while the under surface is frequently provided 
with rhizoidal filaments. ' 

These two families form a transition between the gelatinous, and mostly 
homoiomerous thallus, and the more developed entirely heteromerous thallus 
of much more advanced structure. The fructification in all of them, gelatinous 
and non-gelatinous, is a more or less open apothecium, sometimes immar- 
ginate, and biatorine or lecideine, but often, even in species nearly related 
to these, it is lecanorine with a thalline amphithecium. Rarely are the spori- 
ferous bodies sunk in the tissue, with a pseudo-perithecium, as in Phylliscum. 
It would be difficult to trace advance in all this group on the lines of fruit 
development. The two genera with bright-green gonidia, Psoroma and 
Psoromaria, have been included in Pannariaceae owing to the very close 
affinity of Psoroma hypnorum with Pannaria rubiginosa; they are alike in 
every respect except in their gonidia. Psoromaria is exactly like Psoroma, 
but with immarginate biatorine apothecia, representing therefore a lower 
development in that respect. 

These lichens not only mark the. transition from gelatinous to non- 
gelatinous forms, but in some of them there is an interchange of gonidia. 
The progression in the phylum or phyla has evidently been from blue-green 
up to some highly evolved forms with bright-green algae, though there may 
have been, at the beginning, a substitution of blue-green in place of earlier 
bright-green algae, Phycolichens usurping as itwerethe Archilichen condition. 

e. PELTIGERACEAE AND STICTACEAE. The two families just examined 
marked a great advance which culminated in the lobate aquatic lichen 
Hydrothyria. This lichen, as Sturgis pointed out, shows affinity with other 
Pannariaceae in the structure of the single large-celled cortical layer as well 
as with species of Nepkroma (Peltigeraceae). A still closer affinity may be 
traced with Peltigera in the presence in both plants of veins on the under 
surface. The capacity of Peltigera species to grow in damp situations may 
also be inherited from a form like the submerged Hydrothyria. In both 
families there are transitions from blue-green to bright-green gonidia, or 
vice versa, in related species. Thus in Peltigeraceae we find Peltigera con- 
taining Nostoc in the gonidial zone, with Peltidea which may be regarded 
as a separate genus, or more naturally as a section of Peltigera; it contains 
bright-green gonidia, but has cephalodia containing Nostoc associated with 
its thallus. 

The genus Nephroma is similarly divided into species with a bright-green 
gonidial zone, chiefly Arctic or Antarctic in distribution, and species with 
Nostoc (subgenus Nephromium) more numerous and more widely distributed. 



THE THALLUS 287 

Peltigera and Nephroma are also closely related in the character of the 
fructification. It is a flat non-marginate disc borne on the edge of the 
thallus: in Peltigera on the upper surface, in Nephroma on the under surface. 
The remaining genus Solorina contains normally a layer of bright-green 
algae, but, along with these, there are always present more or fewer Nosloc 
cells, either in a thin layer as in S. crocea or as cephalodia in others, while, 
in three species the algae are altogether blue-green. 

The members of the Peltigeraceae have a thick upper cortex of plecten- 
chyma and in some cases strengthening veins, and long rhizinae on the 
lower side. Some of the species attain a large size, and, in some, soredia 
are formed, an evidence of advance, this being a peculiarly lichenoid form 
of reproduction. 

The Stictaceae form a parallel but more highly organized family, which 
also includes closely related bright-green and blue-green series. They are 
all dorsiventral, but they are mostly attached by a single hold-fast and the 
lobes in some species suggest the fruticose type in their long narrow form. 
A wide cortex of plectenchyma protects both the upper and the lower 
surface and a felt of hairs replaces the rhizinae of other foliose lichens. In 
the genus Sticta (including the section Sticlind) special aeration organs, 
cyphellae or pseudocyphellae, are provided ; in Lobaria these are replaced by 
naked areas which serve the same purpose. 

Nylander 1 regarded the Stictaceae as the most highly developed of all 
lichens, and they easily take a high place among dorsiventral forms, but it 
is generally conceded that the fruticose type is the more highly organized. 
In any case they are the highest reach of the phylum or phyla that started 
with Pyrenopsidaceae and Collemaceae ; the lowly gelatinous thalli changing 
to more elaborate structures with the abandonment of the gelatinous algal 
sheath, as in the Pannariaceae, and with the replacement of blue-green by 
bright-green gonidia. Reinke 2 , considers the Stictaceae as evolved from the 
Pannariaceae more directly from the genus Massalongia. Their relationship 
is certainly with Pannariaceae and Peltigeraceae rather than with Par- 
meliaceae ; these latter, as we shall see, belong to a wholly different series. 

D. EVOLUTION OF ARCHILICHENS 

The study of Archilichens as of Phycolichens is complicated by the 
many different kinds of fungi and algae that have entered into combination ; 
but the two principal types of algae are the single-celled Protococcus group 
and the filamentous Trenlepohtia : as before only the broad lines of thalline 
development will be traced. 

The elementary forms in the different series are of the simplest type a 
somewhat fortuitous association of alga and fungus, which in time bears the 
1 Seep. 126. 2 Reinke 1895. 



288 PHYLOGENY 

lichen fructification. It has been stated that the greatest advance of all 
took place with the formation of a cortex over the primitive granule, 
followed by a restricted area of growth outward or upward which resulted 
finally in the foliose and fruticose thalli. Guidance in following the course 
of evolution is afforded by the character of the fructification, which generally 
shows some great similarity of type throughout the different phyla, and 
remains fairly constant during the many changes of thalline evolution. 
Development starting from one or many origins advances point by point in 
a series of parallel lines. 

a. THALLUSOF PYRENOCARPINEAE. In this series there are two families 
of algae that function as gonidia: Protococcaceae, consisting of single cells, 
and Trentepohliaceae, filamentous. Phyllactidium (Cephaleuros) appears in 
a single genus, Strigula, a tropical epiphytic lichen. 

Associated with these types of algae are a large number of genera and 
species of an elementary character, without any differentiation of tissue. In 
many instances the thallus is partly or wholly embedded in the substratum. 

Squamulose or foliose forms make theirappearance in Dermatocarpaceae : 
in Normandina the delicate shell-like squamules are non-corticate, but in 
other genera, Endocarpon, Placidiopsis, etc., the squamules are corticate and 
of firmer texture, while in Dermatocarpon, foliose fronds of considerable size 
are formed. The perithecial fruits are embedded in the upper surface. 

In only one extremely rare lichen, Pyrenothamnia Spraguei(N. America), 
is there fruticose development: the thallus, round and stalk-like at the base, 
branches above into broader more leaf-like expansions. 

b. THALLUS OF CONIOCARPINEAE. At the base of this series are genera 
and species that are extremely elementary as regards thalline formation, 
with others that are saprophytic and parasitic. The simplest type of thallus 
occurs in Caliciaceae, a spreading mycelium with associated algae (Proto- 
coccaceae) collected in small scattered granules, resembling somewhat a col- 
lection of loose soredia. The species grow mostly on old wood, trunks of trees, 
etc. In Calidwn (Chaenothecd) chrysocephalum as described by Neubner 1 the 
first thallus formation begins with these scattered minute granules; gradually 
they increase in size and number till a thick granular coating of the sub- 
stratum arises, but no cortex is formed and there is no differentiation of tissue. 

The genus Cyphelium (Cypheliaceae) is considered by Reinke to be more 
highly developed, inasmuch as the thalline granules, though non-corticate, 
are more extended horizontally, and, in vertical section, show a distinct 
differentiation into gonidial zone and medulla. The sessile fruit also takes 
origin from the thallus, and is surrounded by a thalline amphithecium, or 
rather it remains embedded in the thalline granule. A closely allied tropical 

1 Neubner 1893. 



THE THALLUS 289 

genus Pyrgillus has reached a somewhat similar stage of development, but 
with a more coherent homogeneous thallus, while in Tylophoron, also tropical 
or subtropical, the fruit is raised above the crustaceous thallus but is thickly 
surrounded by a thalline margin. The alga of that genus is Trentepolilia, 
a rare constituent of Coniocarpineae. 

A much more advanced formation appears in the remaining family 
Sphaerophoraceae. In Calycidium, a monotypic New Zealand genus, the 
thallus consists of minute squamules, dorsiventral in structure but with a 
tendency to vertical growth, the upper surface is corticate and the mazaedial 
apothecia always open are situated on the margins. Tlwlurna dissimilis, 
(Scandinavian) still more highly developed, has two kinds of rather small 
fronds corticate on both surfaces, the one horizontal in growth, crenulate in 
outline, and sterile, the other vertical, about 2 mm. in height, hollow and 
terminating in a papilla in which is seated the apothecium. 

Two other monotypic subtropical genera form a connecting link with 
the more highly evolved forms. In the first, Acroscyphus sphaerophoroides, 
the fronds are somewhat similar to the fertile ones of Tholurna, but they 
possess a solid central strand and the apical mazaedium is less enveloped by 
the thallus. The o\\\er,Pleurocybe madagascarea, has narrow flattish branching 
fronds about 3 cm. in height, hollow in the centre and corticate with marginal 
or surface fruits. 

The third genus, Sphaerophorus, is cosmopolitan ; three of the species are 
British and are fairly common on moorlands, etc. They are fruticose in 
habit, being composed of congregate upright branching stalks, either round 
or slightly compressed and varying in height from about I to 8 cm. The 
structure is radiate with a well-developed outer cortex, and a central strand 
which gives strength to the somewhat slender stalks. The fruits are lodged 
in the swollen tips and are at first enclosed; later, the covering thallus splits 
irregularly and exposes the hymenium. 

Coniocarpineae comprise only a comparatively small number of genera 
and species, but the series is of unusual interest as being extremely well 
defined by the fruit-formation and as representing all the various stages of 
thalline development from the primitive crustaceous to the highly evolved 
fruticose type. With the primitive thallus is associated a wholly fungal 
fruit, both stalk and capitulum, which in the higher forms is surrounded and 
protected by the thallus. Lichen-acids are freely produced even in crustaceous 
forms, and they, along with the high stage of development reached, testify to 
the great antiquity of the series. 

c. THALLUS OF GKAPHIDINEAE. As formerly understood, this series 
included only crustaceous forms with an extremely simple development of 
thallus, fungi and algae whether Palmellaceae, etc., or more frequently 
Trentepohliaceae growing side by side either superficially or embedded in 

s. L. '9 



290 PHYLOGENY 

tree or rock, the presence of the vegetative body being often signalled only 
by a deeper colouration of the substratum. The researches of Almquist, 
and more recently of Reinke and Darbishire, have enlarged our conception 
of the series, and the families Dirinaceae and Roccellaceae are now classified 
in Graphidineae. 

Arthoniaceae, Graphidaceae and Chiodectonaceae are all wholly crus- 
taceous. The first thalline advance takes place in Dirinaceae with two allied 
genera, Dirina and Dirinastrum. Though the thallus is still crustaceous, it 
is of considerable thickness, with differentiation of tissues: on the lower 
side there is a loosely filamentous medulla from which hyphae pierce the 
substratum and secure attachment. Trentepokliagomfaa. lie in a zone above 
the medulla, and the upper cortex is formed of regular palisade hyphae 
forming a " fastigiate cortex." It is the constant presence of Trentepohlia 
algae as well as the tendency to ellipsoid or lirellate fruits that have in- 
fluenced the inclusion of Dirinaceae and Roccellaceae in the series. 

The thallus of Dirinaceae is crustaceous, while the genera of Roccellaceae 
are mostly of an advanced fruticose type, though in one, Roccellina, there is 
a crustaceous thallus with an upright portion consisting of short swollen 
podetia-like structures with apothecia at the tips ; and in another, Roccello- 
grapha, the fronds broaden to leafy expansions. They are nearly all rock- 
dwellers, often inhabiting wind-swept maritime coasts, and a strong basal 
sheath has been evolved to strengthen their foothold. In some genera the 
sheath contains gonidia; in others the tissue is wholly of hyphae in nearly 
every case it is protected by a cortex. 

In the upright fronds the structure is radiate: generally a rather loose 
strand of hyphae more or less parallel with the long axis of the plant forms 
a central medulla. The gonidia lie outside the medulla and just within the 
outer cortex. The latter, in a few genera, is fibrous, the parallel hyphae 
being very closely compacted; but in most members of the family the 
fastigiate type prevails, as in the allied family Dirinaceae. 

d. THALLUS OF CYCLOCARPINEAE. This is by far the largest and most 
varied series of Archilichens. It is derived, as regards the fungal constituent, 
from the Discomycetes, but in these fungi, the vegetative or mycelial body 
gives no aid to the classification which depends wholly on apothecial 
characters. In the symbiotic condition, on the contrary, the thallus becomes 
of extreme importance in the determination of families, genera and species. 
There has been within the series a great development both of apothecial 
and of thalline characters in parallel lines or phyla. 

A A. LECIDEALES. The type of fruit nearest to fungi in form and origin 
occurs in the Lecideales. It is an open disc developed from the fungal sym- 
biont alone, the alga taking no part. There are several phyla to be considered. 



THE THALLUS 291 

aa. COENOGONIACEAE. There are two types of gonidial algae in this 
family, and both are filamentous forms, Trentepohlia in Coenogonium and 
Cladophora in Racodium. The resulting lichens retain the slender thread-like 
form of the algae, their cells being thinly invested by the hyphae and both 
symbionts growing apically. The thalline filaments are generally very 
sparingly branched and grow radially side by side in a loose flat expansion 
attached at one side by a sheath, or the strands spread irregularly over the 
substratum. Plectenchyma appears in the apothecial margin in Coenogonium. 
Fruiting bodies are unknown in Racodium. 

Coenogoniaceae are a group apart and of slight development, only the 
one kind of thallus appearing; the form is moulded on that of the gonidium, 
and is, as Reinke 1 remarks, perfectly adapted to receive the maximum of 
illumination and aeration. 

bb. LECIDEACEAE AND GYROPHORACEAE. The origin of this thalline phylum 
is distinct from that of the previous family, being associated with a different 
type of gonidium, the single-celled alga of the Protococcaceae. 

The. more elementary species are of extremely simple structure as 
exemplified in such species as Lecidea (Biatora) uliginosa or Lecidea granu- 
losa. These lichens grow on humus-soil and the thallus consists of a spreading 
mycelium or hypothallus with more or less scattered thalline granules con- 
taining gonidia, but without any defined structure. The first advance takes 
place in the aggregation and consolidation of such thalline granules and 
the massing of the gonidia towards the light, thus substituting the hetero- 
merous for the homoiomerous arrangement of the tissues. The various 
characters of thickness, areolation, colour, etc. of the thallus are constant and 
are expressed in specific diagnoses. Frequently an amorphous cortex of 
swollen hyphae provides a smooth upper surface and forms a protective 
covering for such long-lived species as Rhizocarpon geographicum, etc. 

The squamulose thallus is well represented in this phylum. The squa- 
mules vary in size and texture but are mostly rather thick and stiff. In 
Lecidea ostreata they rise from the substratum in serried rows forming a 
dense sward; in L. decipiens, also a British species, the squamules are still 
larger, and more horizontal in direction ; they are thick and firm and the 
upper cortex is a plectenchyma of cells with swollen walls. Solitary hyphae 
from the medulla pass downwards into the support. 

Changes in spore characters also arise in these different thalline series, 
as for instance in genera such as Biatorina and Buellia, the one with colour- 
less, the other with brown, two-celled spores. These variations, along with 
changes in the thallus, are of specific or generic importance following the 
significance accorded to the various characters. 

In one lichen of the series, the monotypic Brazilian genus Spltaerophoropsis 
1 Reinke 1895, p. no. 

192 



292 PHYLOGENY 

stereocauloides, the thallus is described by Wainio 1 as consisting of minute 
clavate stalks of interwoven thick-walled hyphae, with gelatinous algae, like 
Gloeocapsa, interspersed in groups, though with a tendency to congregate 
towards the outer surface. 

The highest development along this line of advance is to be found in the 
Gyrophoraceae, a family of lichens with a varied foliose character and dark 
lecideine apothecia. The thallus may be monophyllous and of fairly large 
dimensions or polyphyllous; it is mostly anchored by a central stout hold- 
fast and both surfaces are thickly corticate with a layer of plectenchyma; 
the under surface is mostly bare, but may be densely covered with rhizina- 
like strands of dark hyphae. They are all northern species and rock-dwellers 
exposed to severe extremes of illumination and temperature, but well 
protected by the thick cortex and the dark colouration common to them all. 

cc. CLADONIACEAE. This last phylum of Lecideales is the most interesting 
as it is the most complicated. It possesses a primary, generally sterile, 
thallus which is dorsiventral and crustaceous, squamulose or in some in- 
stances almost foliaceous, along with a secondary thallus of upright radiate 
structure and of very varied form, known as the podetium which bears at 
the summit the fertile organs. 

A double thallus has been suggested in the spreading base, containing 
gonidia, of some radiate lichens such as Roccella, but the upright portion 
of such lichens, though analogous, is not homologous with that of 
Cladoniaceae. 

The algal cells of the family belong to the Protococcaceae. Blue-green 
algae are associated in the cephalodia of Pilophorus and Stereocaulon. 
The primary thallus is a feature of all the members, though sometimes very 
slight and very short-lived, as in Stereocaulon or in the section Cladina of 
the genus Cladonia. Where the primary thallus is most largely developed, 
the secondary (the podetium) is less prominent. 

This secondary thallus originates in two different ways: (i) the primary 
granule may grow upward, the whole of the tissues taking part in the new 
development; or (2) the origin may be endogenous and proceed from the 
hyphae only of the gonidial zone: these push upwards in a compact fascicle, 
as in the apothecial development of Lecidea, but instead of spreading outward 
on reaching the surface, they continue to grow in a vertical direction and 
form the podetium. In origin this is an apothecial stalk, but generally it is 
clothed with gonidial tissue. The gonidia may travel upwards from the 
base or they may possibly be wind borne from the open. The podetium 
thus takes on an assimilative function and is a secondary thallus. 

The same type of apothecium is common to all the genera ; the spores 

1 Wainio 1890. 



THE THALLUS 293 

are colourless and mostly simple, but there are also changes in form and 
septation not commensurate with thalline advance, as has been already noted. 
Thus in Gomphillus, with primitive thallus and podetium, the spores are 
long and narrow with about 100 divisions. 

1. ORIGIN OF CLADONIA. There is no difficulty in deriving Cladoniaceae 
from Lecidea, or, more exactly, from some crustaceous species of the section 
Biatora in which the apothecia as in Cladoniaceae are waxy and more 
or less light-coloured and without a thalline margin. In only a very few 
isolated instances has a thalline margin grown round the Cladonia fruit. 

There are ten genera included in the Cladoniaceae, of which five are 
British. Considerable study has been devoted to the elucidation of develop- 
mental problems within the family by various workers, more especially in the 
large and varied genus Cladonia which is complicated by the presence of 
the two thalli. The family is monophyletic in origin, though many subordi- 
nate phyla appear later. 

2. EVOLUTION OF THE PRIMARY THALLUS. At the base of the series we 
find here also an elementary granular thallus which appears in some species 
of most of the genera. In Gomphillus, a monospecific British genus, the 
granules have coalesced into a continuous mucilaginous membrane. In 
Baeomyces, though mostly crustaceous, there is an advance to the squamulose 
type in B. placophyllus, and in two Brazilian species described by Wainio, 
one of which, owing to the form of the fronds, has been placed in a separate 
genus Hcteromyces. The primary thallus becomes almost foliose also in 
Gymnoderma coccocarpum from the Himalayas, with dorsiventral stratose 
arrangement of the tissues, but without rhizinae. The greatest diversity 
is however to be found in Cladonia where granular, squamulose and almost 
foliose thalli occur. The various tissue formations have already been 
described 1 . 

3. EVOLUTION OF THE SECONDARY THALLUS. Most of the interest 
centres round the development and function of the podetium. In several 
genera the primordium is homologous with that of an apothecium ; its 
elongation to an apothecial stalk is associated with delayed fructification, 
and though it has taken on the function of the vegetative thallus, the purpose 
of elongation has doubtless been to secure good light conditions for the 
fruit, and to facilitate a wide distribution of spores : therefore, not only in 
development but in function, its chief importance though now assimilative 
was originally reproductive. The vegetative development of the podetium is 
correlated with the reduction of the primary thallus which in many species 
bears little relation in size or persistence to the structure produced from it, 
as, for instance, in Cladonia rangiferina where the ground thallus is of the 

1 See Chap. III. 



294 PHYLOGENY 

scantiest and very soon disappears, while the podetial thallus continues to 
grow indefinitely and to considerable size. 

4. COURSE OF PODETIAL DEVELOPMENT. In Baeomyces the podetial 
primordium is wholly endogenous in some species, but in others the 
outer cortical layer of the primary thallus as well as the gonidial hyphae 
take part in the formation of the new structure which, in that case, is simply 
a vertical extension of the primary granule. This type of podetium called 
by Wainio 1 a pseudopodetium also recurs in Pilophorus and in Stereocanlon. 
To emphasize the distinction of origin it has been proposed to classify these 
two latter genera in a separate family, but in that case it would be necessary 
to break up the genus Baeomyces. We may assume that the endogenous 
origin of the "apothecial stalk" is the more primitive, as it occurs in the 
most primitive lecideine lichens, whereas a vertical thallus is always an 
advanced stage of vegetative development. 

Podetia are essentially secondary structures, and they are associated 
both with crustaceous and squamulose primary thalli. If monophyletic in 
origin their development must have taken place while the primary thallus 
was still in the crustaceous stage, and the inherited tendency to form podetia 
must then have persisted through the change to the squamulose type. In 
species such as Cl. caespiticia the presence of rudimentary podetia along 
with large squamules suggests a polyphyletic origin, but Wainio's 1 opinion is 
that such instances may show retrogression from an advanced podetial form, 
and that the evidence inclines to the monophyletic view of their origin. 

The hollow centre of the podetium arises in the course of development 
and is common to nearly all advanced stages of growth. There are how- 
ever some exceptions : in Glossodium aversum, a soil lichen from New 
Granada, and the only representative of the genus, a simple or rarely forked 
stalk about 2 cm. in height rises from a granular or minutely squamulose 
thallus. The apothecium occupies one side of the flattened and somewhat 
wider apex. There is no external cortex and the central tissue is of loose 
hyphae. In Thysanothecium Hookeri, also a monotypic genus from Australia, 
the podetia are about the same height, but, though round at the base, they 
broaden upwards into a leaf-like expansion. The central tissue below is of 
loose hyphae, but compact strands occur above, where the apothecium 
spreads over the upper side. The under surface is sterile and is traversed 
by nerve-like strands of hyphae. 

5. VARIATION IN CLADONIA. It is in this genus that most variation is to 
be found. Characters of importance and persistence have arisen by which 
secondary phyla may be traced within the genus: these are mainly (i) the 
relative development of the horizontal and vertical structures, (2) formation 

1 Wainio 1897. 



THE THALLUS 295 

of the scyphus and branching of the podetium, with (3) differences in colour 
both in the vegetative thallus and in the apothecia. 

Wainio has indicated the course of evolution on the following lines : 
(i) the crustaceous thallus is monophyletic in origin and here as elsewhere 
precedes the squamulose. The latter he considers to be also monophyletic, 
though at more than one point the more advanced and larger foliose forms 
have appeared : (2) the primitive podetium was subulate and unbranched, 
and the apex was occupied by the apothecium. Both scyphus and branching 
are later developments indicating progress. They are in both cases associated 
with fruit-formation scyphi generally arising from abortive apothecia 1 , 
branching from aggregate apothecia. In forms such as Cl. fimbriata, where 
both scyphiferous and subulate sterile podetia are frequent, the latter (sub- 
species fibula) are retrogressive, and reproduce the ancestral pointed pode- 
tium. (3) In subgen. Cenomyce, with a squamulose primary thallus, there is 
a sharp division into two main phyla characterized by the colour of the 
apothecia, brown in Ochrophaeae the colour being due to a pigment and 
red in Cocciferae where the colouring substance is a lichen-acid, rhodocladonic 
acid. In the brown-fruited Ochrophaeae there are again several secondary 
phyla. Two of these are distinguished primarily by the character of the 
branching : (a) the Chasmariae in which two or several branches arise from 
the same level, entailing perforation of the axils (Cl. furcata, Cl. rangi- 
formis, Cl. squamosa, etc.), the scyphi also are perforated. They are further 
characterized by peltate aggregate apothecia, this grouping of the apothecia 
according to Wainio being the primary cause of the complex branching, 
the several fruit stalks growing out as branches. The second group (&), the 
Clausae, are not perforated and the apothecia are simple and broad-based 
on the edge of the scyphus (Cl. pyxidata, Cl. fimbriata, etc.), or on the tips 
of the podetia (Cl. cariosa, Cl. leptophylla, etc.). A third very small group 
also of Clausae called (c} Foliosae has very large primary squamules and 
reduced podetia (Cl. foliacea, etc.), while finally (d) the Ochroleucae, none of 
which is British, have poorly developed squamules and variously formed 
yellowish podetia with pale-coloured apothecia. 

The Cocciferae represent a phylum parallel in development with the 
Ochrophaeae. The species have perhaps most affinity with the Clausae, the 
vegetative thallus both the squamules and the podetia being very much 
alike in several species. Wainio distinguishes two groups based on a differ- 
ence of colour in the squamules, glaucous green in one case, yellowish in 
the other. 

6. CAUSES OF VARIATION. External causes of variation in Cladonia are 
chiefly humidity and light, excess or lack of either effecting changes which 
may have become fixed and hereditary. Minor changes directly traceable 

1 See Chap. III. 



296 PHYLOGENY 

to these influences are also frequent, viz. size of podetia, proliferation and the 
production more or less of soredia or of squamules on the podetia, though 
only in connection with species in which these variations are already an 
acquired character. The squamules on the podetium more or less repeat 
the form of the basal squamules. 

/. PODETIAL DEVELOPMENT AND SPORE-DISSEMINATION. In a recent 
paper by Hans Sattler 1 the problem of podetial development in Cladonia 
is viewed from a different standpoint. He holds that as the podetia are 
apothecial stalks, their service to the plant consists in the raising of the 
mature fruit in order to secure a wide distribution of the spores, and that 
changes in the form of the podetium are therefore but new adaptations for 
the more efficient discharge of this function. 

Following out this idea he regards as the more primitive forms those in 
which both the spermogonia, as male reproductive bodies, and the carpogonia 
occur on the primary thallus, ascogonia and trichogynes being formed before 
the podetium emerges from the thallus. Fertilization thus must take place 
at a very early period, though the ultimate fruiting stage may be long 
delayed. Sattler considers that any doubt as to actual fertilization is without 
bearing on the question, as sexuality he holds must have originally existed 
and must have directed the course of evolution in the reproductive bodies. 
In this primitive group, called by him the "Floerkeana" group, the podetia 
are always short and simple, they are terminated by the apothecium and 
no scyphi are formed (Cl. Floerkeana, Cl. leptophylla, Cl. cariosa, Cl. caespi- 
ticia, Cl. papillaria, etc.). 

In his second or "pyxidata" group, he places those species in which the 
apothecia are borne at the edge of a scyphus. That structure he follows 
Wainio in regarding as a morphological reaction on the failure of the first 
formed apical apothecium: it is, he adds, a new thallus in the form of a 
spreading cup and bears, as did the primary thallus, both the female primordia 
and the spermogonia. In some species, such as Cl. foliacea, there may be 
either scyphous or ascyphous podetia, and spermogonia normally accompany 
the carpogonium appearing accordingly along with it either on the squamule 
or on the scyphus. 

As the pointed podetia are the more primitive, Sattler points out that 
they may reappear as retrogressive structures, and have so appeared in the 
"pyxidata" group in such species as Cl. fimbriata. He refers to Wainio's 
statement that the abortion of the apothecium being a retrogressive anomaly, 
while scyphus formation is an evolutionary advance, the scyphiferous species 
present the singular case, "that a progressive transmutation induced by 
a retrogressive anomaly has become constant." 

1 Sattler 1914. 



THE THALLUS 297 

His third group includes those forms that grow in crowded tufts or 
swards such as Cl. rangiferina, Cl.furcata, Cl. gracili s, etc. They originate, 
as did the pyxidata group, in some Floerkeana-\\\i& form, but in the "rangi- 
ferina" group instead.of cup-formation there is extensive branching. In the 
closely packed phalanx of branches water is retained as in similar growths 
of mosses, and moist conditions necessary for fertilization are thus secured 
as efficiently as by the water-holding scyphus. 

Sattler in his argument has passed over many important points. Above 
all he ignores the fact that whatever may have been the original nature 
and function of the podetium, it has now become a thalline structure and 
provides for the vegetative life of the plant, and that it is in its thalline 
condition that the many variations have been formed ; the scyphus is not, 
as he contends, a new thallus, it is only an extension of thalline characters 
already acquired. 

8. PILOPHORUS, STEREOCAULON AND ARGOPSIS. These closely related 
genera are classified with Cladonia as they share with it the twofold thallus 
and the lecideine apothecia. The origin of the podetium being different 
they may be held to constitute a phylum apart, which has however taken 
origin also from some Biatora form. 

The primary thallus is crustaceous or minutely squamulose and the 
podetia of Pilophorus, which are short and unbranched (or very sparingly 
branched), are beset with thalline granules. The podetia of Stereocaulon 
and Argopsis are copiously branched and are more or less thickly covered 
with minute variously divided leaflets. Cephalodia containing blue-green 
algae occur on the podetia of these latter genera; in Pilophorus they are 
intermixed with the primary thallus. 

The tissue systems are less advanced in these genera than in Cladonia : 
there, is no cortex present either in Pilophorus or in Argopsis or in some 
species of Stereocaulon, though in others a gelatinous amorphous layer 
covers the podetia and also the stalk leaflets. The stalks are filled with 
loose hyphae in the centre. 

BB. LECANORALES. This second group of Cyclocarpineae is distinguished 
by the marginate apothecium, a thalline layer providing a protecting amphi- 
thecium. The lecanorine apothecium is of a more or less soft and waxy 
consistency, and though the disc is sometimes almost black, neither hypo- 
thecium nor parathecium is carbonaceous as in Lecidea. The affinity of 
Lecanora is with sect. Biatora, and development must have been from a 
biatorine form with a persistent thallus. The margin or amphithecium 
varies in thickness: in some species it is but scanty and soon excluded by 
the over-topping growth of the disc, so that a zone of gonidia underlying 
the hypothecium is often the only evidence of gonidial intrusion left in 
fully formed fruits. 



298 PHYLOGENY 

The marginate apothecium has appeared once and again as we have 
seen. It is probable however that its first development was in this group of 
lichens, and even here there may have been more than one origin as there 
is certainly more than one phylum. 

aa. COURSE OF DEVELOPMENT. At the base of the series, the thallus is 
of the crustaceous type somewhat similar to that of Lecidea, but there are 
none of the very simple primitive forms. Lecanora must have originated 
when the crustaceous lecideine thallus was already well established. Its 
affinity is with Lecidea and not with any fungus: where the thallus is 
evanescent or scanty, its lack is due to retrogressive rather than to primitive 
characters. 

bb. LECANORACEAE.. A number of genera have arisen in this large family, 
but they are distinguished mainly if not entirely by spore characters, and 
by some systematists have all been included in the one genus Lecanora, 
since the changes have taken place within the developing apothecium. 

There is one genus, Harpidium, which is based on thalline characters,, 
represented by one species, H. rutilans, common enough on the Continent, 
but not yet found in our country. It has a thin crustaceous homoiomerous 
thallus, the component hyphae of which are divided into short cells closely 
packed together and forming a kind of cellular tissue in which the algae are 
interspersed. The dorsiventral stratose arrangement prevails however in the 
other genera and a more or less amorphous " decomposed " cortex is fre- 
quently present. The medulla rests on the substratum. 

With the stouter thallus, there is slightly more variety of crustaceous 
form than in Lecideaceae: there occurs occasionally an outgrowth of the 
thalline granules as in Haematomma ventosum which marks the beginning 
of fruticulose structure. Of a more advanced structure is the thallus of 
Lecanora esculenta, a desert lichen which becomes detached and erratic, and 
which in some of its forms is almost coralline, owing to the apical growth of 
the original granules or branches: a more or less radiate arrangement of 
the tissues is thus acquired. 

The squamulose type is well represented in Lecanora, and the species 
with that form of thallus have frequently been placed in a separate genus, 
Squamaria. These squamules are never very large; they possess an upper, 
somewhat amorphous, cortex; the medulla rests on the substratum, except 
in such a species as Lecanora lentigera, where they are free, a sort of fibrous 
cortex being formed of hyphae which grow in a direction parallel with the 
surface. In none of them are rhizinae developed. 

cc. PARMELIACEAE. The chief advance, apart from size, of the squamulose 
to the foliose type is the acquirement of a lower cortex along with definite 
organs of attachment which in Parmeliaceae are invariably rhizoidal and 



THE THALLUS 299 

are composed of compact strands of hyphae extending from the cells of 
the lower cortex. 

In the genus Parmelia rhizinae are almost a constant character, though 
in a few species, such as Parmelia physodes, they are scanty or practically 
absent. It is not possible, however, to consider that these species form a 
lower group, as in other respects they are highly evolved, and rhizinae may 
be found at points on the lower surface where there is irritation by friction. 
Soredia and isidia occur frequently and, in several species, almost entirely 
replace reproduction by spores. In one or two northern or Alpine species, 
P. stygia and P.pubescens, the lobes are linear or almost filamentous. They 
are retained in Parmelia because the apothecia are superficial on the fronds 
which are partly dorsiventral, and because rhizinae have occasionally been 
found. Some of the Parmeliae attain to a considerable size ; growth is centri- 
fugal and long continued. 

Two monotypic genera classified under Parmeliaceae, Physcidia and 
Heterodea, are of considerable interest as they indicate the bases of parallel 
development in Parmelia and Cetraria. The former, a small lichen, is corti- 
cate only on the upper surface, and without rhizinae; and from the description, 
the cortex is of a fastigiate character. The solitary species grows on bark 
in Cuba; it is related to Parmelia, as the apothecia are superficial on the 
lobes. The second, Heterodea Mulleri, a soil-lichen from Australasia, is more 
akin to Cetraria in that the apothecia are terminal. The upper surface is 
corticate with marginal cilia, the lower surface naked or only protected by 
a weft of brownish hyphae amongst which cyphellae are formed ; pseudo- 
cyphellae appear in Cetraria. 

The genus Cetraria contains very highly developed thalline forms, either 
horizontal (subgenus Platysma), or upright (Eiicetraria}. Rhizinae are scanty 
or absent, but marginal cilia in some upright species act as haptera. Cetraria 
aculeata is truly fruticose with a radiate structure. 

An extraordinary development of the under cortex characterizes the 
genera Anzia 1 and Pannoparmelia: rhizinae-like strands formed from the 
cortical cells branch and anastomose with others till a wide mesh of a 
spongy nature is formed. They are mostly tropical or subtropical or Austra- 
lasian, and possibly the spongy mass may be of service in retaining moisture. 
A species of Anzia has been recorded by Darbishire 2 from Tierra del Fuego. 

dd. USNEACEAE. As we have seen, the change to fruticose structure 
has arisen as an ultimate development in a number of groups; it reaches 
however its highest and most varied form in this family. Not only are there 
strap-shaped thalli, but a new form, the filamentous and pendulous, appears; 
it attains to a great length, and is fitted to withstand severe strain. The 

1 See p. 90. * Darbishire 1912. 



3 oo PHYLOGENY 

various adaptations of structure in these two types of thallus have already 
been described 1 . 

In Parmelia itself there are indications of this line of development in 
P. stygia, with short stiff upright branching fronds, and in P. pubescens, 
with its tufts of filaments, but these two species are more or less dorsiventral 
in structure and do not rise from the substratum. In Cetraria also there 
is a tendency towards upright growth and in C. aculeata even to radiate 
structure. But advance in these directions has stopped short, the true line 
of evolution passing through species like Parmelia physodes with raised, and 
in some varieties, tubular fronds, and the somewhat similar species P. Kamt- 
schadalis with straggling strap-like lobes, to Evernia. That genus is a true 
link between foliose and fruticose forms and has been classified now with 
one series, now with the other. 

In Evernia furfuracea, the lobes are free from the substratum except 
when friction causes the development of a hold-fast and the branching out 
of new lobes from that point. It is however dorsiventral in structure, the 
under surface is black and the gonidial zone lies under the upper cortex. 
Evernia prunastri is white below and is more fruticose in habit, the long 
fronds all rising from one base. They are thin and limp, no strengthening 
tissue has been evolved, and they tend to lie over on one side; both surfaces 
are corticate and gonidia sometimes travel round the edge, becoming fre- 
quently lodged here and there along the under side. 

The extreme of strap-shaped fruticose development is reached in the 
genus Ramalina. In less advanced species such as R. evernioides there is a 
thin flat expansion anchored to the substratum at one point and alike on 
both surfaces. In R.fraxinea the fronds may reach considerable width (var. 
ampliata), but in that and in most species there is a provision of sclerotic 
strands to support and strengthen the fronds. One of those best fitted to 
resist bending strains is R. scopulorum (siliquosd) which grows by preference 
on sea-cliffs and safely withstands the maximum of exposure to wind or 
weather. 

The filamentous structure appears abruptly, unless we consider it as 
foreshadowed by Parmelia pubescens. The base is secured by strong sheaths 
of enduring character; tensile strains are provided for either by a chondroid 
axis, as in Usnea, or by cortical development, as in Alectoria; the former 
method of securing strength seems to be the most advantageous to the plant 
as a whole, since it leaves the outer structures more free to develop, and there 
is therefore in Usnea a greater variety of branching and greater growth in 
length, which are less possible with the thickened cortex of Alectoria. 

ee. PHYSCIACEAE. There remains still an important phylum of Lecano- 
rales well defined by the polarilocular spores 2 . It also arises from a Biatora 
1 See p. 101. 2 See p. 188. 



THE THALLUS 301 

species and forms a parallel development. Even in this phylum there are two 
series : one with colourless spores and mostly yellow or reddish either in 
thallus or apothecium, and the other with brown spores and with cinereous- 
grey or brown thalli. The dark spores are in many of the species typically 
polarilocular, though in some the median septum is riot very wide and no 
canal is visible. Practically all of the lighter coloured forms contain parietin 
either in thallus or apothecia or in both ; it is absent in the dark-spored series. 

Among the lighter coloured forms it is difficult to decide which of these 
two striking characteristics developed first, the acid or the peculiar spore. 
Probably the acid has the priority: there is one common rock lichen in this 
country, Placodium rupestre (Lecanora irrubata\ which gives a strong red 
acid reaction with potash, but in which the spores are still simple, and the 
'fruit structure in the biatorine stage. Another species, PI. luteoalbum, with a 
purplish reaction in the fruit only, shows septate spores but with only a 
rather narrow septum. The development continues through biatorine forms 
to lecanorine with a fully formed thalline margin. Among these latter we 
encounter PI. nivale which is well provided with acid but in which the spores 
have become long and fusiform with little trace of the polar cells or central 
canal. We must allow here also for reversions, and wanderings from the 
straight road. 

From crustaceous the advance is normal and simple to squamulose forms 
which in this phylum maintain a stiff regularity of thalline outline termed 
"effigurate"; the squamules, developing from the centre, extend outwards in a 
radiate-stellate manner. There are also foliose thalli in the genus Xanthoria 
and fruticose in Teloschistes. The cortex in the former horizontal genus is of 
plectenchyma, and no peculiar structures have emerged. In Teloschistes the 
cortex is of compact parallel hyphae (fibrous) which form the strengthening 
structure of the narrow compressed fronds (T.flavicans}. 

In the brown-spored series there is a considerable number of species that 
are crustaceous united in the genus Rinodina, all of which have marginate 
apothecia. One of them, Rinodina oreina, approaches in thalline structure the 
effigurate forms of Placodium; while in R. isidioides, a rare British species, 
there is an isidioid squamulose development. 

Among foliose genera, the tropical genus Pyxine is peculiar in its almost 
lecideine fruit, a few gonidia occurring only in the early stages; its affinity 
with Physcia holds, however, through the one-septate brown spores with very 
thick walls and the reduced lumen of the cells. The more simple type of 
fruit may be merely retrogressive. 

Pliyscia, the remaining genus, is mainly foliose and with dorsiventral 
thallus. A few species have straggling semi-upright fronds and these have 
sometimes been placed in a separate genus Anaptychia. Only one " Anap- 
tychia" Ph. intricata, has a radiate structure with fibrous cortex all round ; 
in the others the upper cortex alone is fibrous of long parallel hyphae 



3 02 



PHYLOGENY 



but that character appears in nearly every one of the horizontal species as 
well, sometimes in the upper, sometimes in the lower cortex. 

In Physcia the horizontal thallus is of smaller dimensions than in Par- 
melia, and never becomes so free from the substratum: it is attached by 
rhizinae and soredia appear frequently. Very often the circular effigurate 
type of development prevails. 

It is difficult to trace with any certainty the origin of this series of the 
phylum. Some workers have associated it with the purely lecideine genus, 
Buellia, but the brown septate spores of the latter are of simple structure, 
though occasionally approaching the Rinodina type. There are also 
differences in the thallus, that of Buellia, especially when it is saxicolous, 
inclining to Rhizocarpon in form. It is more consistent with the outer and 
inner structure to derive Rinodina from some crustaceous Placodium form 
with a marginate apothecium, therefore from a form of fairly advanced 
development. As the parietin content disappeared perhaps from the pre- 
ponderance of other acids the colouration changed and the spores became 
dark-coloured. 

Many genera and even families, such as Thelotremaceae, etc., have 
necessarily been omitted from this survey of phylogeny in lichens, but the 
tracing of the main lines of development has indicated the course of evolu- 
tion, and has demonstrated not only the close affinity between the members 
of this polyphyletic class of plants, as shown in the constantly recurring 
thalline types, but it has proved the extraordinary vigour gained by both 
the component organisms through the symbiotic association. 

The principal phyla 1 , developing on somewhat parallel lines, are given in 
the appended table : 

ARCHILICHENS 



Phyla 


Crustose 


Squamulose 


Foliose 


Fruticose 


Pyrenolichens 
Coniocarpineae 


Verrucariaceae 
Caliciaceae 


Dermato 
Sphaero 


carpaceae 
jhoraceae 


Sphaerophoraceae 


TArthoniaceae 


| 




Graphidineae \ Graphidaceae 






Roccellaceae 




^Dirinaceae 








Cyclocarpineae 












ILecideaceae Gyrophoraceae 






Coenogoniaceae 






Lecideales 


(filamentous gonidia) 








Cladoniaceae 




. 


Lecanorales 


(primary and secondary thalli) 
Lecanoraceae 


Parmeliaceae 


Usneaceae 


Polariloculares 










{Colourless spores 


Placodium 


Xanthoria 


Teloschistes 


Brown spores 


Rinodina, Pyxine 




Physcia j Physcia (Anaptychia) 



1 Dr Church (1920) has published a new conception of the origin of lichens. See postscript at 
the end of the volume, p. 421. 



THE THALLUS 



303 



SCHEME OF SUGGESTED PROGRESSION IN LICHEN STRUCTURE 



PYRENOCARPINEAE 



CONIOCARPINEAE 



PYRENOCARPEAE 



CONIOCARPEAE 



Pyrenothamniaceae 
Dermatocarpaceae 
Dermatocarpaceae 

Verrucariaceae 
(Protococcaceae) 


Phyllopy 

Pyreni 
(Trent 


reniaceae 

laceae 
-Pohtia) 


Sphaerophorus 
I Pilophorus 

Pleurocybe Acroscyphus 
Calycidium Tholurna 


Cyphelium Tylophoron Tylophorella 
i , 1 
Caliciaceae Pyrgillus 
(Protococcaceae) (Trentepoklia) 



CYCLOCARPINEAE 
PHYCOLICHENS (CYANOPHILI) 







Stictaceae 








1 


Peltigeraceae 






l_ 


Hydrothyria 


Peccania Phloeopeccania 


Pannariaceae 






Leptodendriscum and 


1 




Synalissa 
Thyrea 1 


Leptogidium 
Polychidium 


1 
Heppiaceae 




P 'r 




Leptogium 




Pauha 1 




| 




L_J 




Collema 




Pyrenopsidaceae Thermutis 


Physma 




(Gloeocapsci) 


. (Scytonsma) 


(No*toc) 





LECIDEALES 



LECANORALES POLARILOCULARES 



Usneae 

Stereocaulon Eucetraria Ramalina 
Ochropheae Cocciferae 

Evernia Teloschistes 



Gyrophoraceae 



Cladbnia Pilophoro 




Cetraria Parmelia 
(Platysma) physodes 
Parme- 



Xantnoria 



Physcia 

sect. 
Anaptychia 



Physcia 



{Sect. Psora 
Sect. Eulecidea 



Heterodea Physcidia lEuplacodium 



^ecanora sect. -I \ ICallopisma 

Squamaria JPlacodium \ Rinod 

i I | IBUstenU | 

Lecanora Colourless spores 



Sect. Biatora 
I 



Lecidea 
(Protococcaceae) 



CHAPTER VIII 

SYSTEMATIC 
I. CLASSIFICATION 

A. WORK OF SUCCESSIVE SYSTEMATISTS 

SINCE the time when lichens were first recognized as a separate class 
as members of the genus Lichen by Tournefort 1 or as "Musco-fungi" 
by Morison 2 , many schemes of classification have been outlined, and the 
history of the science of lichenology, as we have seen, is a record of attempts 
to understand their puzzling structure, and to express that understanding 
by relating them to each other and to allied classes of plants. The great 
diversity of opinion in regard to their affinities is directly due to their 
composite nature. 

a. DILLENIUS AND LINNAEUS. The first systematists were chiefly im- 
pressed by their likeness to mosses, hepatics or algae. Dillenius* in the 
Historia Muscorum grouped them under the moss genera: IV. Usnea, 
V. Coralloides and VI. Lichenoides. Linnaeus 4 classified them among algae 
under the general name Lichen, dividing them into eight orders based on 
thalline characters in all but one instance, the second order being distin- 
guished from the first by bearing scutellae. The British botanists of the 
latter part of the eighteenth century Hudson, Lightfoot and others were 
content to follow Linnaeus and in general adopted his arrangement. 

b. ACHARIUS. Early in the nineteenth century Acharius, the Swedish 
Lichenologist, worked a revolution in the classification of lichens. He gave 
first place to the form of the thallus, but he also noted the fundamental 
differences in fruit-formation: his new system appeared in the Methodus 
Lichenum 5 with an introduction explaining the terms he had introduced, 
many of them in use to this day. 

Diagnoses of twenty-three genera are given with their included species. 
The work was further extended and emended in Lichenographia Uni- 
versalis* and in the Synopsis Lichenum 1 . In his final arrangement the 
family "Lichenes" is divided into four classes, three of which are charac- 
terized solely by apothecial characters; the fourth class has no apothecia. 
They are as follows : 

Class I. Idiothalami with three orders, Homogenei, Heterogenei and Hyperogenei : 
the apothecia differ in texture and colouration from the thallus: Lecidea, Opegrapha, 
Gyrophora, etc. 

Class II. Coenothalami, with three orders, Phymaloidei, Discoidei and Cephaloidei. 
1 Tournefort 1694. 2 Morison 1699. 3 Dillenius 1741. 4 Linnaeus 1753. 

6 Acharius 1803. 6 Acharius 1810. 7 Acharius 1814. 



FAMILIES AND GENERA 305 

The apothecia are partly formed from the thallus: Lecanora, Parme/ta, etc. The 
Pyrenolichens are also included by him in this class, because "the thallus surrounds and 
is concrete with the partly or wholly immersed apothecia." 

Class III. Homothalami with two orders, Scutellati and Peltati. The apothecia are 
formed from the cortical and medullary tissue of the thallus : Ramalina, Usnea, Collema, 
etc. 

Class IV. Athalami, with but one sterile genus, Lepraria. 

The orders are thus based on the form of the fruit; the genera in the 
Synopsis number 41. Large genera such as Lecanora with 132 species are 
divided into sections, many of which have in turn been established as 
genera, by S. F. Gray in 1821, and later by other systematists. 

The Synopsis was the text-book adopted by succeeding botanists for 
some 40 years with slight alterations in the arrangement of classes, genera, 
etc. 

Wallroth 1 and Meyer 2 followed with their studies on the lichen thallus, 
and Wallroth's division into "Homoiomerous" and "Heteromerous" was 
accepted as a useful guide in the maze of forms, representing as it did 
a great natural distinction. 

c. SCHAERER. This valiant lichenologist worked continuously during 
the first half of the nineteenth century, but with very partial use of the 
microscope. His last publication in 1850, an Enumeration of Swiss Lichens, 
was the final declaration of the older school that relied on field characters. 
His classification is as follows : 

Class I. Lichenes Discoidei, with ten orders from Usneacei to Graphidei ; fruits 
open. 

Class II. Lichenes Capitati, with three orders: Calicioidei, Sphaerophorei and Cla- 
doniacei ; fruits stalked. 

Class III. Lichenes Verrucarioidei, with three orders: Verrucarii, Pertusarii and 
Endocarpei : fruits closed. 

An "Appendix" contains descriptions of Crustacei and Fruticulosi, all 
sterile forms, except Coniocarpon and Arthonia, which seem out of place, 
and finally a "Corollarium" of gelatinous lichens all classified under one 
genus Collema. 

d. MASSALONGO AND KOERBER. As a result of their microscopic 
studies, these two workers proposed many changes based on fruit and 
spore characters, and Koerber in the Systema Lichenum Germaniae (1855) 
gave expression to these views in his classification. He also made use of 
Wallroth's distinctions of "homoiomerous" and "heteromerous," thus dividing 
lichens at the outset into those mostly with blue-green and those with bright- 
green gonidia. 

1 Wallroth 1825. * Meyer ' 82 5- 



3 o6 SYSTEMATIC 

The following is the main outline of Koerber's classification: 

Series I. Lichenes Heteromerici. 

Order I. Lich. Thamnoblasti (fruticose). 

Order II. Lich. Phylloblasti (foliose). 

Order III. Lich. Kryoblasti (crustaceous). 
Series II. Lichenes Homoeomerici. 

Order IV. Lich. Gelatinosi. 

Order V. Lich. Byssacei. 

With the exception of Order V all are subdivided into two sections, 
"gymnocarpi" with open fruits and "angiocarpi" with closed fruits, a 
distinction that had long been recognized both in lichens and in fungi. 

e. NYLANDER. The above writers had been concerned with the inter- 
relationships of lichens ; Nylander, who was now coming forward as a 
lichenologist of note, gave a new turn to the study by dwelling on their 
relation to other classes of plants. Without for a moment conceding that 
they were either algal or fungal, he yet insisted on their remarkable affinity 
to algae on the one hand, and to fungi on the other, and he sought to make 
evident this double connection by his very ingenious scheme of classfication 1 . 
He began with what we may call "algal lichens," those associated with 
blue-green gonidia in the family "Collemacei"; he continued the series to 
the most highly evolved foliose forms and then wound up with those that 
are most akin to fungi, that is, those with least apparent thalline formation 
according to him the " Pyrenocarpei." 

In his scheme, which is the one followed by Leighton and Crombie, the 
"family" represents the highest division; series, tribe, genus and species 
come next in order. We have thus : 
Fam. I. Collemacei. 

Fam. II. Myriangiacei (now reckoned among fungi). 
Fam. III. Lichenacei. 

This last family, which includes the great bulk of lichens, is divided into 
the following series: I. Epiconiodei; II. Cladoniodei; III. Ramalodei ; 
IV. Phyllodei; V. Placodei; VI. Pyrenodei. It is an ascending series up 
to the Phyllodei, or foliaceous lichens, which he considers higher in develop- 
ment than the fruticose or filamentous Ramalodei. The Placodei include 
four tribes on a descending scale, the Lecanorei, Lecidinei, Xylographidei 
and Graphidei. The classification is almost wholly based on thalline form, 
except for the Pyrenodei in which are represented genera with closed fruits, 
there being one tribe only, the Pyrenocarpei. 

Nylander claims however to have had regard equally to the reproductive 
system and was the first to give importance to the spermogonia. The 
classification is coherent and easy to follow, though, like all classifications 

1 Nylander 1854. 



FAMILIES AND GENERA 307 

based on imperfect knowledge, it is not a little artificial ; also while magnify- 
ing the significance of spermogonia and spermatia, he overlooked the much 
more important characters of the ascospores. 

/. Mt)LLER(-ARGAU). In preparing his lists of Genevan lichens (1862), 
Miiller realized that Nylander's series was unnatural, and he found as he 
studied more deeply that lichens must be ranged in parallel or convergent 
but detached groups. He recognized three main groups : 

1. Eulichens, divided into Capitularieae, Discocarpeae and Verru- 

caroideae. 

2. Epiconiaceae. 

3. Collemaceae. 

He suggested that, in relation to other plants, Eulichens approach 
Pezizae, Hysteriaceae and Sphaeriaceae; Epiconiaceae have affinity with 
Lycoperdaceae, while Collemaceae are allied to the algal family Nosto- 
caceae. These three groups of Eulichens, he held, advanced on somewhat 
parallel lines, but reached a very varied development, the Discocarpeae 
attaining the highest stage of thalline form. M tiller accepted as characters 
of generic importance the form and structure of the fruiting body, the 
presence or absence of paraphyses, and the septation, colour, etc. of the spores. 
A few years later (1867) the composite nature of the lichen thallus was 
announced by Schwendener, and, after some time, was acknowledged by 
most botanists to be in accordance with the facts of nature. Any system 
of classification, therefore, that claims to be a natural one, must, while 
following as far as possible the line of plant development, take into account 
the double origin of lichens both from algae and fungi, the essential unity 
and coherence of the class being however proved by the recurring similarity 
between the thalline types of the different phyla. As Muller had surmised: 
"they are a series of parallel detached though convergent groups." 

g. REINKE. The arrangement of Ascolichens on these lines was first 
seriously studied by Reinke 1 , and his conclusions, which are embodied 2 in 
the Lichens of Schleswig-Holstein, have been largely accepted by succeeding 
workers. He recognizes three great subclasses: I. Coniocarpi; 2. Disco- 
carpi; 3. Pyrenocarpi. 

The Coniocarpi are a group apart, but as their fruit is at first entirely 
closed at least in some of the genera the more natural position for them 
is between Discocarpi and Pyrenocarpi. It is in the arrangement of the 
Discocarpi that variation occurs. Reinke's arrangement of orders and 
families in that subclass is as follows : 

Subclass 2. Discocarpi. 

Order I. GRAMMOPHORI: Fam. GRAPHIDACEI and XYLOGRAPHACEI. 
1 Reinke 1894, '95, '96. 2 Darbishire and Fischer- Benzon 1901. 

20 2 



3 o8 SYSTEMATIC 

Order II. LECIDEALES: Fam. GYALECTACEI, LECIDEACEI, UMBILICARI- 

ACEI and CLADONIACEI. 
Order III. PARMELIALES: Fam. URCEOLARIACEI, PERTUSARIACEI, PAR- 

MELIACEI, PHYSCIACEI, TELOSCHISTACEI and ACAROSPORACEI. 
Order IV. CYANOPHILI: Fam. LICHINACEI, EPHEBACEI, PANNARIACEI, 

Sr/CTACf, PELTIGERACEI, COLLEMACEI and OMPHALARIACEI. 

The orders represent generally the principal phyla or groups, the families 
subordinate parallel phyla within the orders. The first three orders are 
stages of advance as regards fruit development ; the Cyanophili are a group 
apart. 

Wainio 1 rendered great service to Phylogeny in his elaborate work on 
Cladoniaceae, the most complicated of all the lichen phyla. He also drew 
up a scheme of arrangement in his work on Brazil Lichens 2 . There is in 
it some divergence from Reinke's arrangement, as he tends to give more 
importance to the thallus than to fruit characters as a guide. He places, for 
instance, Gyrophorei beside Parmelei and at a long distance from his Lecidei. 
The Cyanophili group of families he has interpolated between Buelliae 
(Physciaceae) and Lecideae. Many workers approve of Wainio's classifica- 
tion but it presents some difficult problems. 

h, ZAHLBRUCKNER. The systematist of greatest weight in recent times 
is A. Zahlbruckner, who is responsible for the systematic account of lichens 
in Engler and Prantl's Naturlichen Pflanzenfamilien. It is difficult to 
express the very great service he has rendered to Lichenology, in that and 
other world-wide studies of lichens. The sketch of lichen phylogeny as 
given in the present volume owes a great deal to the sound and clear 
guidance of his work, though his conclusions may not always have been 
accepted. The classification in the Pflanzenfamilien is the one now gene- 
rally followed. 

The class Lichenes is divided by Zahlbruckner 3 into two subclasses, 
I. Ascolichens and II. Hymenolichens. He gives a third class, Gastero- 
lichens 4 , but as it was founded on error 5 , it need not concern us here. The 
Ascolichens are by far the more important. These are subdivided into: 

Series I. PYRENOCARPEAE, with perithecial fruits. 
Series 2. GYMNOCARPEAE, with apothecial fruits. 

These are again broken up into families, and in the arrangement and 
sequence of the families Zahlbruckner indicates his view of development 
and relationship. They occur in the following order: 

1 Wainio 1887, '94, '97. 2 Wainio 1890. 3 Zahlbruckner 1907. 

4 Massee 1887. 5 Fischer 1890. 



FAMILIES AND GENERA 309 

SERifcs i. PYRENOCARPEAE 

ALGAL CELLS PROTOCOCCACEAE OR PALMELLA. 

MORIOLACEAE \ 

EPIGLOEACEAE \, Thallus crustaceous, perithecia solitary 

VERRUCARIACEAE] 

DERMATOCARPACEAE. Thallus squamulose or foliose. 

P YRENO THA MNIA CEA E. Thallus fruticose. 

ALGAL CELLS PRASIOLA. 
VI. MASTOIDIACEAE. 

ALGAL CELLS TRENTEPOHLIA. 
VII. PYRENULACEAE } 
VIII. PARATHELIACEAEY Thallus crustaceou s, perithecia occurring singly. 

IX. TRYPE^HELIACEAE\ . 

X. ASTROTHELIACEAE}' Thallus crustaceous, perithecia united (stromatoid). 

J XI. MYCOPORACEAE. Thallus crustaceous, perithecia in compact groups with a 

common outer wall. 

XII. PHYLLOPYREN1ACEAE. Thallus minutely foliose. 
ALGAL CELLS PHYLLACTIDIUM OR MYCOIDEA. 

XIII. STRIGULACEAE. Tropical leaf-lichens. ,,*-. 
ALGAL CELLS NOSTOC OR SCYTONEMA. 

XIV. PYRENIDIACEAE. Thallus minutely squamulose or fruticose. 

SERIES 2. GYMNOCARPEAE 

Subseries i. Coniocarpineae, with subperithecial fruits. 
Subseries 2. Graphidineae, with elongate, narrow fruits. 
Subseries 3. Cyclocarpineae, with round open fruits. 

SUBSERIES i. CONIOCARPINEAE 

This is a well-defined group, peculiar in the disappearance of the asci at an early stage 
so that the spores lie like a powder in the globose partly closed fruits. Algal cells, bright- 
green ; Protococcaceae. There are only three families : 

XV. CALICIACEAE. Thallus crustaceous, apothecia stalked. 

XVI. CYPHELIACEAE. Thallus crustaceous, apothecia sessile. 

XVII. SPHAEROPHORACEAE. Thallus foliose or fruticose, apothecia sessile. 

SUBSERIES 2. GRAPHIDINEAE 

This subseries comes next in the form of fruit development ; generally the apothecia 
are elongate, with a narrow slit-like opening, so that a transverse section shows almost a 
perithecial outline. Algal cells are mostly Trentepohlia. 
XVII!. ARTHONIACEAE. Thallus crustaceous, apothecia oval or linear, flat. 

XIX. GRAPHIDACEAE. Thallus crustaceous, apothecia linear, raised. 

XX. CHIODECTONACEAE. Thallus crustaceous, apothecia generally immersed in 

a stroma. 

DIRINACEAE. Thallus crustaceous, corticate above, apothecia round. 
ROCCELLACEAE. Thallus fruticose, apothecia round or elongate. 

SUBSERIES 3. CYCLOCARPINEAE 

A large and very varied group ! In most of the families the algal cells are bright-green 
(Chlorophyceae), in some they are blue-green (Cyanophyceae), these latter corresponding 
to Reinke's order Cyanophili. The apothecia, as the name implies, are round and open ; 
the "Cyanophili" have been placed by Zahlbruckner after those families in which the 



3io 



SYSTEMATIC 



XXIII. 
XXIV. 
XXV. 



> XXVI. 
XXVII. 



XXVIII. 



apothecium has no thalline margin. They form a phylum distinct from those that precede 
and those that follow. 

The first family of the Cyclocarpineae, the Lecanactidaceae, is often placed under 
Graph idineae; in any case it forms a link between the two subseries. 

i. Lecideine group (apothecia without a thalline margin). 

LECANACTIDACEAE. Thallus crustaceous. Algal cells Trentepohlia. 
Apothecium with carbonaceous hypothecium or parathecium. 

PILOCARPACEAE. Thallus crustaceous. Algal cells Protococcaceae. Apo- 
thecia with a dense rather dark hypothecium. 

CHRYSOTHRICACEAE. Thallus felted, loose in texture. Algal cells Pal- 
mella^ Protococcaceae or Trentepohlia. Apothecia with or without a thalline 
margin. The affinity of the "Family" seems to be with Pilocarpaceae. 

\ Thallus crustaceous. Algal cells in the first Tren- 

THELOTREMACEAE [ tepohlia; in the second Protococcaceae. In both 

DIPLOSCHISTAChAE( there are prominent double margins round the 
' apothecium. 

ECTOLECHIACEAE. Thallus very primitive in type. Algal cells Proto- 
coccaceae. Apothecia with or without a thalline margin. Nearly related to 
Chrysothricaceae. 

GYALECTACEAE. Thallus crustaceous. Algal cells Trentepohlia, Phyllac- 
tidium or rarely Scytonema. Apothecia biatorine, i.e. of soft consistency and 
without gonidia. 

COENOGONIACEAE. Thallus confusedly filamentous (byssoid). Algal cells 
Trentepohlia or Cladophora. Apothecia biatorine. 

LECIDEACEAE. Thallus crustaceous or squamulose. Algal cells Proto- 
coccaceae. Apothecia biatorine (soft), or lecideine (carbonaceous). 

PHYLLOPSORACEAE. Thallus squamulose or foliose. Algal cells Proto- 
coccaceae. Apothecia biatorine or lecideine. 

CLADONIACEAE. Thallus twofold. Algal cells Protococcaceae. Apo- 
thecia biatorine or lecideine. 

GYROPHORACEAE. Thallus foliose. Algal cells Protococcaceae. Apo- 
thecia lecideine. 

ACAROSPORACEAE. Thallus primitive crustaceous, squamulose or foliose. 
Algal cells Protococcaceae. Apothecia with or without a thalline margin ; 
very various, but always with many-spored asci. 

2. Cyanophili group. 

In this group the classification depends almost entirely on the nature of the algal 
constituents. The apothecia are in most genera provided with a thalline margin. 

a. More or less gelatinous when moist. 

XXXVI. EPHEBACEAE. Algal cells Scytonema or Stigonema. Thallus minutely 

fruticose or filamentous. 

XXXVII. PYRENOPSIDACEAE. Algal cells Gloeocapsa (Gloeocapsa, Xanthocapsa 

or Chroococcus}. Thallus crustaceous, minutely foliose or fruticose. 
XXXVIII. LICHINACEAE. Algal cells Rivularia. Thallus crustaceous, squamulose 

or minutely fruticose. 

XXXIX. COLLEMACEAE. Algal cells Nostoc. Thallus crustaceous, minutely fruti- 
cose, or squamulose to foliose. 

XL. HEPPIACEAE. Algal cells Scytonema. Thallus generally squamulose 
and formed of plectenchyma. 



XXX. 

XXXI. 
XXXII. 
XXXIII. 
XXXIV. 
XXXV. 



FAMILIES AND GENERA 311 

b. Not gelatinous when moist. 
^~\ XLl. PANNARIACEAE. Algal cells Nostoc, Scytonema or rarely bright-green, 

Protococcaceae. Thallus crustaceous, squamulose or foliose. 
XLll. STICTACEAE. Algal cells Nostoc or Protococcaceae. Thallus foliose, 

and very highly developed, corticate on both surfaces. 

XLIII. PELTIGERACEAE. Algal cells Nostoc or Protococcaceae. Thallus 
foliose, corticate above. 

3. Lecanorine group (apothecia with a thalline margin). 

The remaining families have all bright-green gonidia and nearly always apothecia 
with a thalline margin. The group includes several distinct phyla : 

XLIV. PERTUSARIACEAE. Thallus crustaceous. Apothecia, one or several 

immersed in thalline tubercles ; spores mostly very large. 
XLV. LECANORACEAE. Thallus crustaceous or squamulose. Apothecia mostly 

superficial. 
XLVI. PARMELIACEAE. Thallus foliose, rarely almost fruticose or filamentous. 

Apothecia scattered over the surface or marginal, sessile. 
' XLVI I. USNEACEAE. Thallus fruticose or filamentous. Apothecia sessile or 

shortly stalked. 

N] XLVI 1 1. CALOPLACACEAE. Thallus crustaceous, squamulose or minutely fruti- 
cose. Apothecia with polarilocular colourless spores. 
XLIX. TELOSCHISTACEAE. Thallus foliose or fruticose. Apothecia with 

polarilocular colourless spores. 

L. BUELLIACEAE. Thallus crustaceous or squamulose. Apothecia (lecideine 

or lecanorine) with two-celled, thick-walled brown spores (polarilocular in 
part). 

LI. PHYSCIACEAE. Thallus foliose, rarely partly fruticose. Apothecia with 
two-celled thick-walled brown spores (polarilocular in part). 

Subclass 2. Hymenolichens. 

There are only three closely related genera of Hymenolichens, Cora, Corella and 
Dictyonema with Chroococcus or Scytonema algae. 

There is reason to dissent from the arrangement in one or two instances which will 
be pointed out in the following examination of families and genera. 



B. FAMILIES AND GENERA OF ASCOLICHENS 

The necessity for a well-reasoned and well-arranged system of classifica- 
tion is self-evident: without a working knowledge of the plants that are 
the subject of study no progress can be made. The recognition of plants 
as isolated individuals is not sufficient, it must be possible to place them in 
relation to others; hence the importance of a natural system. In identifying 
species artificial aids, such as habitat and substratum, are also often of great 
value, and a good working system should take account of all characteristics. 

Lichen development is the result of two organisms mutually affecting 
each other, but as the fungus provides the reproductive system, it is the 
dominant partner : the main lines of classification are necessarily determined 



3 i2 SYSTEMATIC 

by fruit characters. The algae occupy a subsidiary position, but they also 
are of importance in shaping the form and structure of the thallus. The 
different phyla are often determined by the presence of some particular alga ; 
it is in the delimitation of families that the algal influence is of most effect. 

Zahlbruckner's system gives due weight to the inheritance from both 
fungus and alga with, however, the fungus as the chief factor in development, 
and as his work is certain to be generally followed by modern lichenologists, 
it is the one of most immediate interest. His scheme has been accepted in 
the following more detailed account of families and genera, and for the 
benefit of home workers those that have not so far been recorded from the 
British Isles have been marked with an asterisk. 

It cannot be affirmed that nomenclature is as yet firmly established in 
lichenology. Both on historical grounds and on those of convenience, the 
subject is one of extreme importance, and interest in it is one of the main 
avenues by which we secure continuity with the past, and by which we are 
able to realize not only the difficulty, but the romance of pioneer work. 
Besides, there can be no exchange of opinion between students nor assured 
knowledge of plants, until the names given to them are beyond dispute. 
According to the ruling of the Brussels Botanical Congress in 1910, 
Linnaeus's 1 list of lichens in the Species Plantarum has been selected as the 
basis of nomenclature, but since his day many new families, genera and 
species have been described and often insufficiently delimited. It is not 
easy to decide between priority, which appeals to the historical sense, and 
recent use which is the plea of convenience. Here also it seems there can 
be no rigid decision; the one aim should be to arrive at a conclusion 
satisfactory to all, and accepted by all. 

In the following necessarily brief account of families and genera, the 
"spermogonia" or "pycnidia" have in most cases been left out of account, 
as in many instances they vary within the family and occasionally even 
within the genus. Their taxonomic value is not without importance, but, 
in the general systematic arrangement, they are only subsidiary characters. 
An account of them has already been given, and for more detailed state- 
ments the student is referred to purely systematic works. 

There are two main types of spore production in the "pycnidia" which 
have been shortly described by Steiner 2 as "exobasidial" and "endobasidial." 
In the former the sporophores are simple or branched filaments, at the 
apices of which a short process grows out and buds off a pycnidiospore; 
in the latter the spores are budded directly from cells lining the walls or* 
filling the cavity of the pycnidium. The exobasidial type is more simply 
rendered in the following pages by "acrogenous," the endobasidial by 
"pleurogenous" spore production. In many cases the "spermogonia" or 
1 Linnaeus 1753. 2 Steiner 1901. 



FAMILIES AND GENERA 313 

"pycnidia" are still imperfectly known. In designating the gonidial algae, 
the more comprehensive Protococcaceae has been substituted for Protococcus, 
as in many cases the alga is* probably not Protococcus as now understood, 
but some other genus of the family 1 . 

SUBCLASS I. ASCOLICHENS 
SERIES I. PYRENOCARPINEAE 

It is on mycological grounds that Pyrenocarpineae are placed at the 
base of lichen classification. There is no evidence that the series was first 
in time. 

I. MORIOLACEAE 

This family was described by Norman 2 in 1872 from specimens col- 
lected by himself in Norway or in the Tyrol, on soil or more frequently on 
trees. There seems to have been no further record, and Zahlbruckner, 
while accepting the family, suggests that an examination or revision may 
be necessary. 

The thallus is crustaceous. The algal cells, Protococcaceae, occur either 
in groups (sometimes stalked) surrounded by a plectenchymatous wall and 
called by Norman "goniocysts," or they form nests in the thallus termed 
"nuclei" which are surrounded by a double wall of plectenchyma, colourless 
in the interior and brown outside. Norman invented the term "Allelositis- 
mus," which , may be rendered "mutualism," to indicate this peculiar form 
of thallus. The species of Spheconisca are fairly numerous on poplars, willows 
and conifers: 

Algae in ; 'goniocysts" i. *Moriola Norm. 3 

Algae in double-walled "nuclei" ... 2. *Spheconisca Norm. 

II. EPIGLOEACEAE 

The family consists of but one genus and one species, Epigloea bactrospora, 
and, according to Zahlbruckner, further examination is necessary to make 
certain as to the lichenoid nature of the plant. 

Zukal 4 found the perithecia scattered over the leaves of mosses, and he 
alleges that hyphae connected with the perithecium were closely associated 
with the alga, Palmella botryoides, and were causing it no harm. Along with 
the perithecia he also found minute pycnidia. The "thallus" is of a gelatinous 
nature and homoiomerous in structure; the perithecia are soft and clear- 
coloured with many-spored asci and colourless one-septate spores. 

The small globose pycnidia contain simple sporophores and acrogenous 
straight or slightly bent rod-like spores. 

Asci many-spored ; spores one-septate, i. *Epigloea Zukal. 

1 See p. 56. 2 Norman 1872 and '74. 

3 Genera marked with an asterisk have not been found in the British Isles. 4 Zukal 1890. 



3 i 4 SYSTEMATIC 

III. VERR UCARIA CEA E 

In all the genera of this family the thallus is crustaceous, and, with very 
few exceptions, the species are saxicolous or terricolous. The thallus is 
variable within the crustaceous limits, and may be superficial and very 
conspicuous, almost imperceptible, or wholly immersed in the substratum. 
The algal cells are Protococcaceae, and in two of the genera the green cells 
penetrate the hymenium and grow in rows alongside of the asci. The 
perithecia are small roundish structures scattered over the thallus, the base 
immersed, but the upper portion generally projecting. An outer dark- 
coloured wall surrounds the whole perithecium (entire) or only the upper 
exposed portion (dimidiate) ; it opens above by a pore or ostiole more or 
less prominent. 

In some of the genera the paraphyses become dissolved at an early 
stage, and somewhat similar filaments near the ostiole, termed periphyses, 
aid in the expulsion of the spores. The spores vary in septation, colour 
and size, and these variations have served to delimit the genera which 
have been formed from the original very large genus Verrucaria. The ascus 
may be 1-2-, 4- or 8-spored. In only one genus is it many-spored 
( Trimmatothele). 

The genera are as follows : 

Perithecia with simple ostioles. 

Paraphyses disappearing early, or wanting. 

Spores simple, ellipsoid I. Verrucaria Web. 

Spores simple, elongate vermiform 2. Sarcopyrenia Nyl. 

Spores simple, numerous in the ascus 3. *Trimmatothele Norm. 

Spores i-3-septate 4. Thelidium Massal. 

Spores murifbrm (with transverse and longitudinal divisions). 

Without hymenial gonidia 5. Polyblastia Massal. 

With hymenial gonidia 6. Staurothele Norm. 

Paraphyses present. 
Spores simple. 

Without hymenial gonidia 7. Thrombium Wallr. 

With hymenial gonidia 8. *Thelenidia Nyl. 

Spores 3-septate, broadly ellipsoid 9. *Geisleria Nitschke. 

Spores acicular, many-septate 10. Gongylia Koerb. 

Spores muriform u. Microglaena Lonnr. 

Perithecia with a wide ring round the ostiole. 

Spores muriform; paraphyses unbranched 12. *Aspidothelium Wain. 

Spores elongate, many-septate; paraphyses branched 13. *Aspidopyrenium Wain. 

IV. DERMATOCARPACEAE 

In this family there is a much more advanced thalline development 
generally squamulose or with some degree of foliose structure, though in 
the genus Endocarpon, some of the species are little more than crustaceous. 



FAMILIES AND GENERA 315 

The gonidia are bright-green Protococcaceae (according to Chodat, Cocco- 
botrys in Dermatocarpori). In Endocarpon they appear in the hymenium. 

The least developed in structure is Normandina : the thallus of the 
single species consists of delicate shell-like squamules which are non- 
corticate above and below. In the other genera there is a cortex of 
plectenchyma. 

The perithecia are almost wholly immersed, and open above by a straight 
ostiole. The fructification of Dacampia is considered by some lichenologists 
to be only a parasite on the white thickish squamulose thallus with which 
it is associated. 

Hymenial gonidia present. 

Spores muriforrn I. Endocarpon Hedw. 

Hymenial gonidia absent. 

Thallus non-corticate 2. Normandina Wain. 

Thallus corticate. 

Spores simple, colourless 3. Dermatocarpon Eschw. 

Spores simple, brown 4. *Anapyrenium Miill.-Arg. 

Spores elongate-septate, colourless 5. *Placidiopsis Beltr. 

Spores elongate-septate, brown 6. *Heterocarpon Miill.-Arg. 

Spores muriforrn, colourless 7. *Psoroglaena Miill.-Arg. 

Spores muriforrn, brown 8. Dacampia Massal. 

V. PYRENOTHAMNIACEAE 

Thallus more or less fruticose and corticate on both surfaces. Algal 
cells Protococcaceae. 

Only two genera are included in this family : Nylanderiella with one 
species from New Zealand, with a small laciniate thallus up to 15 mm. in 
height, partly upright, partly decumbent, and attached to the substratum by 
basal rhizinae ; the other small genus, Pyrenothamnia, belongs to N. America ; 
the thallus has a short rounded stalk which expands above to an irregular 
frond. The perithecia are immersed in the fronds. 

Spores colourless, i-septate i. *Nylanderiella Hue 1 . 

Spores brown, muriforrn 2. *Pyrenothamnia Tuckerm. 

VI. MASTOIDEACEAE 

A family containing one genus and one species, with a wide distribution, 
having been found in Siberia, on the Antarctic continent (Graham's Land), 
as also in Tierra del Fuego, South Georgia, South Shetland Islands and 
Kerguelen. The thallus is foliose, of small thin lobes, and without rhizinae. 
Algal cells Prasiola-. The perithecia are globose and partly project from 
the thallus; the asci are 8-spored; the paraphyses are mucilaginous and 
partly dissolving. 

Spores elongate-fusiform, simple, colourless ...i. *Mastoidea Hook, and Harv. 
1 Hue 1914. 2 Hue 1909. 



3 i6 SYSTEMATIC 

VII. PYRENULACEAE 

This family of crustaceous lichens differs from Verrucariaceae chiefly in 
the gonidium which is a species of Trentepohlia. Genera and species are 
largely corticolous and the thallus is inconspicuous, often developing within 
the substratum. The perithecia, like those of Verrucariae, are immersed or 
partly emergent and have an entire or dimidiate outer wall. They are 
scattered over the thallus except in Anthracothecium where they are often 
coalescent. This genus is tropical or subtropical except for one species 
which inhabits S.W. Ireland. 

Paraphyses are variable, and in some species tend to disappear, but do 
not dissolve in mucilage. The spores are generally colourless, only in one 
monotypic genus, C&ccotrema, are they simple. The cells into which the 
spore is divided differ in form according to the genus. 
Paraphyses branched and entangled or wanting. 

Perithecia opening above by stellate lobes i. *Asteroporum Miill.-Arg. 

Perithecia opening by a pore. 
Spores variously septate. 

Spore cells cylindrical or cuboid. 

Spores colourless, elongate or ovate i-5-septate 2. Arthopyrenia Massal. 

Spores colourless, filiform I -multi-septate 3. Leptorhaphis Koerb. 

Spores colourless, muriform 4. Polyblastropsis A. Zahlbr. 

Spores brown, ovoid or elongate 2-5-septate ... 5. Microthelia Koerb. 
Spore cells globose or lentiform, 3-multi-septate 6. *Pseudopyrenula Miill.-Arg. 
Paraphyses unbranched free. 

Spore cells cylindrical or cuboid. 

Perithecia beset with hairs 7. *Stereochlamys Miill.-Arg. 

Perithecia naked. 

Asci disappearing ; spores elongate multi- 
septate, colourless 8. *Belonia Koerb. 

Asci persistent. 

Spores simple, ellipsoid, colourless 9. *Coccotrema Miill.-Arg. 

Spores elongate, i-multi-septate, colourless... 10. Porina Miill.-Arg. 

Spores elongate, i -multi -septate, brown u. Blastodesmia Massal. 

Spores muriform, colourless 12. *Clathroporina Miill.-Arg. 

Spores elongate, 2-3-septate, colourless 13. Thelopsis Nyl. 

Spore cells globose or lentiform. 

Spores elongate, i-5-septate, brown 14. Pyrenula Massal. 

Spores muriform, brown 15. Anthracothecium Massal. - 

VIII. PARATHELIACEAE 

This family is peculiar in that the perithecia open by a somewhat 
elongate ostiole that slants at an oblique angle. The algal cells are Trente- 
pohlia. Genera and species are endemic in tropical or subtropical regions 
of the Western hemisphere, though a species of Pleurotrema has been found 
in subantarctic America. They are corticolous and the thallus is either 



FAMILIES AND GENERA 317 

superficial or embedded. The genera are arranged according to spore 
characters : 

Spores elongate, 2- or more-septate. 

Spore cells cylindrical, colourless i. *Pleurotrema Miill.-Arg. 

Spore cells globose-lentiform. 

Spores colourless 2. *Plagiotrema Miill.-Arg. 

Spores brown 3. *Parathelium Miill.-Arg. 

Spores muriform. 

Spores colourless 4. *Campylothelium Mull.-Arg. 

Spores brown 5. *Pleurothelium Miill.-Arg. 

IX. TR YPE THEL I A CEA E 

This and the following two families are distinguished by the pseudo- 
stroma or compound fruit, a character rare among lichens, though the true 
stroma is frequent in Pyrenomycetes in such genera as Dothidea, Valsa, etc. 
The genera are crustaceous and corticolous and occur with few exceptions 
in tropical or subtropical regions, mostly in the Western Hemisphere. 
Several grow on officinal bark (Cinchona, etc.). Algal cells are Trentepohlia. 
As in many tropical lichens, the spores are large. The genera are based 
chiefly on spore characters, on septation, and on the form of the spore 
cells : 

Spore cells cylindrical or cuboid. 

Spores colourless, elongate, multi-septate i. *Tomasiella Miill.-Arg. 

Spores colourless, muriform 2. *Laurera Rehb. 

Spores brown, muriform 3. *Bottaria Massal. 

Spore cells globose-lentiform. 

Spores colourless, elongate, multi-septate 4. *Tr\ pethelium Spreng. 

Spores brown, elongate, multi-septate 5. Melanotheca Miill.-Arg. 

X. ASTROTHELIACEAE 

The perithecia are either upright or inclined, and occur usually in 
radiate groups. They are free or united in a stroma, and the elongate 
ostioles open separately or coalesce in a common canal. The genera are 
all crustaceous, with Trentepohlia gonidia. They are tropical or subtropical, 
mostly in the Western Hemisphere; but species of Parmentaria and Astro- 
thelium have been recorded also from Australia. 

The spores are all many-celled and the form of their cells is a generic 
character : 

Spores elongate, multi-septate. 

Spore cells cylindrical I- *Lithothelium Miill.-Arg. 

Spore cells globose-lentiform. 

Spores colourless 2. *Astrothelium Trev. 

Spores brown 3- *Pyrenastrum Eschw. 

' Spores muriform. 

Spores colourless 4- *Heufleria Trev. 

Spores brown 5- *Parmentaria Fe'e. 



3 i8 SYSTEMATIC 

XL MYCOPORACEAE 

A small family with only two genera which are found in both Hemi- 
spheres ; species of both occur in Great Britain. They are all corticolous. 
The perithecia are united into a partially chambered fruiting body surrounded 
by a common wall, but opening by separate ostioles. The thallus is thinly 
crustaceous, with Palmella gonidia in Mycoporum, and Trentepohlia in 
Mycoporellum. The spores are colourless or brown in both genera : 

Spores muriform i. Mycoporum Flot. 

Spores elongate, multi-septate 2. Mycoporellum A. Zahlbr. 

XII. PHYLLOPYRENIACEAE 

Thallus foliose with both surfaces corticate and attached by rhizinae. 
Algal cells Trentepohlia. There is but one genus, Lepolichen, which has a 
laciniate somewhat upward growing thallus. Two species, both from South 
America, have been described, L. granulatus Miill.-Arg. and L. coccophora 
Hue. The latter has been recently examined by Hue 1 who finds, on the 
thalli, cephalodia which are peculiar in containing bright-green gelatinous 
algae either Urococcus or Gloeocystis, one of the few instances known of 
chlorophyllaceous algae forming part of a cephalodium. Gloeocystis may be 
the only alga present in the cephalodium ; Urococcus is always accompanied 
by Scytonema. 

The perithecia are immersed in thalline tubercles : 
Spores colourless, simple, ovoid or ovoid-elongate I. *Lepolichen Trevis. 

XIII. STRIGULACEAE 

A family of epiphyllous lichens inhabiting and disfiguring coriaceous 
evergreen leaves, or occasionally fern leaves in tropical or subtropical regions. 
The algae associated are Mycoidea and Phycopeltis (Phyllactidium). The 
only truly parasitic lichen, Strigula, belongs to this family: the alga precedes 
the lichen on the leaves and is gradually invaded by the hyphae of the 
lichen and altered in character. The small black perithecia are scattered 
over the surface. In Strigula the lichen retains the spreading rounded form 
of the alga. The other genera are more irregular. 

Thallus orbicular in outline I. *Strigula Fries. 

Thallus irregular. 

Perithecia without hairs. 
Spores colourless. 

Spores elongate, multi-septate 2. *Phylloporina Mull.-Arg. 

Spores muriform 3. *Phyllobathelium Miill.-Arg. 

Spores brown. 

Spores simple 4. *Haplopyrenula Miill.-Arg. 

Spores elongate, [-3-septate 5. *Microtheliopsis Miill.-Arg. 

Perithecia beset with stiffhairs 6. *Trichothelium Miill.-Arg. 

1 Hue 1905. 



FAMILIES AND GENERA 319 

XIV. PYRENWIACEAE 

The only family of Pyrenocarpineae associated with blue-green algae. 
The genera of Pyrenidiaceae are all monotypic, only one is common and 
of wide distribution, Coriscium (Normandina Nyl.). Pyrenidium is the only 
member that has a fruticose thallus, and that is of minute dimensions. 
Eolichen Heppii, found and described by Zukal, is a doubtful lichen. " Lopho- 
thelium " Stirton is a case of parasitism of a fungus, Ticothecium, on the 
squamules of Stereocaulon condensatum. 

Algal cells Scytonema or Stigonema. 

Thallus crustaceous 1 ; spores simple, colourless i. *Rhabdopsora Mull.-Arg. 

Thallus crustaceous ; spores i-septate, colourless 2. *Eolichen Zuk. 

Thallus crustaceous ; spores muriform, brown 3. *Pyrenothrix Riddle 2 . 

Thallus squamulose ; spores numerous, simple 4. *Placothelium Miill.-Arg. 

Algal cells Nostoc. 

Thallus crustaceous; spores filiform, simple, colourless 5. *Hassea A. Zahlbr. 

Thallus fruticose ; spores elongate, 3-septate, brown ...6. Pyrenidium Nyl. 

Algal cells Microcystis (Polycoccus). 
Thallus squamulose ; fructification unknown 7. Coriscium Wainio. 

SERIES II. GYMNOCARPEAE 

SUBSERIES i. Coniocarpineae 

This small subseries is marked by the peculiar "mazaedium" type of 
fruit with its disappearing asci. It forms a connecting link between the 
families with perithecia and those with apothecia. The thallus is crustaceous 
or fruticose, often poorly developed and sometimes absent. The algal cells 
are Protococcaceae or rarely Trentepohlia. 

XV. CALICEACEAE 

The thallus is thinly crustaceous, sometimes brightly coloured, some- 
times absent, taking no part in the formation of the fruits; these have 
upright stalks with a small capitulum, and often look like minute nails. 
One genus, Sphinctrina, is parasitic on the thallus of other lichens, mostly 
Pcrtusariae. 

Fruits with slender stalks. 
Spores simple. 

Spores colourless i. Coniocybe Ach. 

Spores brown 2. Chaenotheca Th. Fr. 

Spores septate, brown. 

Spores i-septate 3. Calicium De Not. 

Spores 3-7-septate 4- Stenocybe Nyl. 

Fruits with short thick stalks. 

Spores globose, brown (parasitic) 5- Sphinctrina Fries. 

Spores i-septate, brown 6. *Pyrgidium Nyl. 

1 Zahlbr., in Hedwigia, LIX. p. 301, 1917. * Riddle 1917. 



3 20 SYSTEMATIC 

XVI. CYPHELIACEAE 

Thallus crustaceous. Algal cells Protococcaceae or Trentepohlia. Apo- 
thecia sessile, more widely open than in the previous family; in some genera 
the thallus forms an outer apothecial margin. The genera Farriola from 
Norway and Tylophorella from New Granada are monotypic. The British 
genus Cyphelium has been known as Trachytia. 

Thallus with Protococcaceae. 

Spores colourless, simple i. *Farriola Norm. 

Spores brown, i-3-septate (rarely simple or muriform) ...2. - Cyphelium Th. Fr. 
Thallus with Trentepohlia. 

Spores simple, many in the ascus 3. *Tylophorella Wainio. 

Spores 8 in the ascus. 

Apothecia with a thalline margin 4. *Tylophoron Nyl. 

Apothecia without a thalline margin 5. *Pyrgillus Nyl. 

XVII. SPHAEROPHORACEAE 

The most highly evolved family of the subseries, as regards the thallus. 
Algal cells Protococcaceae. In Tkolurna, a small lichen endemic in Scan- 
dinavia, there is a double thallus : one of horizontal much-divided squa- 
mules, the other swollen, upright, terminating in the capitulum. The fruit 
is lateral in Calycidium, a squamulose form from New Zealand, and in 
Pleurocybe from Madagascar, with stiff strap-shaped fronds. All the genera 
are monotypic except Sphaerophorus, of which genus ten species are recorded, 
some of them with a world-wide distribution. The spores are brown and 
simple or I -septate. 

Thallus squamulose and upright i. *Tholurna Norm. 

Thallus wholly squamulose 2. *Calycidium Stirton. 

Thallus fruticose. 

Fronds hollow in the centre 3. *Pleurocybe Miill.-Arg. 

Fronds not hollow. 

Fruit without a thalline margin 4. *Acroscyphus Lev. 

Fruit inclosed in the tip of the fronds 5. Sphaerophorus Pers. 

SUBSERIES i. Graphidineae 

In this subseries are included five families that differ rather widely from 
each other both in thallus and apothecia; the latter are more or less 
carbonaceous and mostly with a proper margin only. Families and genera 
are widely distributed, though most abundant in warm regions. Algal cells 
mostly Trentepohlia. 

A comprehensive study of the apothecia of this series by Bioret 1 gives 
some interesting results in regard to the paraphyses: in Arthonia they are 
irregular in direction and much-branched ; in Opegrapha, the paraphyses 
are vertical and parallel with more regular branching ; Stigmatidium (Entero- 

1 Bioret 1914. 



FAMILIES AND GENERA 321 

graplia} resembles Opegrapha in this respect as does also Platygrapha, a 
genus of Lecanactidaceae, while in Grapliis the paraphyses are vertical, 
unbranched and free; Melaspilea paraphyses are somewhat similar to those 
of Gr aphis. 

XVIII. ARTHONIACEAE 

The thallus of Arthoniaceae is corticolous with few exceptions and is 
very inconspicuous, being largely embedded in the substratum. The 
apothecia (ardellae) are round, irregular or stellate, without any margin, 
the hymenium being protected by the dense branching of the paraphyses 
at the tips. 

A rthonia is abundant everywhere. The species of the other genera belong 
mostly to tropical or subtropical countries. Arthoniopsis is similar to 
Arthonia in the character of the fruits, but the gonidium is a Phycopeltis, 
and it is only found on leaves. SynartJionia with peculiar stromatoid fruc- 
tification is monotypic; it occurs in Costa Rica. 

Thallus with Trentepohlia gonidia. 
Apothecia scattered. 

Spores elongate i- or pluri-septate i. Arthonia Ach. 

Spores muriform 2. Arthothelium Massal. 

Apothecia stromatoid. 

Spores elongate, multi-septate 3. *Synarthonia Mull.-Arg. 

Thallus with Pahnella gonidia. 

Spores i- or more-septate 4. Allarthonia Nyl. 

Spores muriform 5- *Allarthothelium Wain. 

Thallus with Phycopeltis gonidia. 
Spores elongate I- or more-septate 6. *Arthoniopsis Miill.-Arg. 

XIX. GRAPHIDACEAE 

Thallus crustaceous, inconspicuous, partly immersed, mainly growing 
on bark but occasionally on dead wood or stone. Algal cells chiefly 
Trentepohlia, very rarely Palniella or Phycopeltis (epiphyllous). Apothecia 
(lirellae) carbonaceous more or less linear, opening by a narrow slit with 
a well-developed proper margin except in Gymnographa, a monotypic 
Australian genus. In two genera, the fruit is of a compound nature, several 
parallel discs occurring in one lirella: these are Ptychographa (on bark in 
Scotland) and Diplogramma (Australia), both are monotypic. They must 
not be confused with Graphis elegans and allied species in which the sterile 
carbonaceous margin is furrowed. Two tropical genera associated with 
Phycopeltis are epiphyllous. 

Graphidaceae are among the oldest recorded lichens, attention having 
been drawn to them since early times by the resemblance of the lirellae on 
the bark of trees to hieroglyphic writing. 



322 SYSTEMATIC 

Thallus with Palmetto, gonidia. 
Apothecia single. 

Hypothec) urn dark-brown. 

Spores simple i. Lithographa Nyl. 

Hypothecium colourless or brownish. 
Spores colourless. 

Spores simple 2. Xylographa Fries. 

Spores elongate 3-8-septate 3. *Aulaxina Fee. 

Spores brown. 

Spores i -septate 4. Encephalographa Massal. 

Spores pluri-septate, then muriform 5. *Xyloschistes Wain. 

Apothecia compound. 

Spores simple, colourless 6. Ptychographa Nyl. 

Spores pluri-septate, colourless 7. *Diplogramma Miill.-Arg. 

Thallus with Trentepohlia gonidia. 

Spores elongate i -multi-septate, the cells longer than wide. 
Spores brown. 

Spores i-(rarely more)-septate 8. Melaspilea Nyl. 

Spores 3-septate (apothecia rudimentary) 9. *Gymnographa Miill.-Arg. 

Spores colourless. 

Spores acicular, coiled (many in the ascus) 10. *Spirographa A. Zahlbr. 

Spores fusiform, straight n. Opegrapha Humb. 

Spores muriform. , 

Spores elongate, central cells finally muriform 12. *Dictyographa Miill.-Arg. 
Spores elongate, septate, cells wider than long. 
Paraphyses unbranched, filiform. 

Spores multi-septate, colourless 13. Graphis Adans. 

Spores multi-septate, brown 14. Phaeographis Miill.-Arg. 

Spores muriform, colourless 15. Graphina Miill.-Arg. 

Spores muriform, brown 16. Phaeographina Miill.-Arg. 

Paraphyses clavate, warted at tips 17. *Acanthothecium Wain. 

Paraphyses branched, interwoven above 18. *Helminthocarpon Fe"e. 

Thallus with Phycopeltis gonidia (epiphyllous). 

Spores elongate, 3-9-septate, colourless 19. *Opegraphella Miill.-Arg. 

Spores elongate, i-septate, brown 20. *Micrographa Miill.-Arg. 

XX. CHIODECTONACEAE 

Specially distinguished in this subseries by the grouping of the somewhat 
rudimentary apothecia in pseudostromata in which they are almost wholly 
immersed. In form they are roundish or linear; the spores are septate or 
muriform. The thallus is thinly crustaceous and continuous : in Glyphis, 
Sarcographa and Sarcographina there is an amorphous upper cortex, the 
other genera are non-corticate. Algal cells are Trentepohlia with the 
exception of two epiphyllous genera associated with Phycopeltis. 

Genera and species are mostly tropical. Sderophyton with five species 
is represented in Europe by a single British specimen, S. circumscriptum. 

The form of the paraphyses is a distinguishing character of the genera. 



FAMILIES AND GENERA 323 

Thallus with Trentepohlia gonidia. 
Paraphyses free, unbranched. 

Spore cells short or almost globose. 

Spores elongate, multi-septate, colourless i. Glyphis Fe"e. 

Spores elongate, multi-septate brown 2. *Sarcographa Fe"e. 

Spores muriform, brown 3. *Sarcographina Miill.-Arg. 

Spore cells longer and cuboid. 

Spores muriform, colourless 4. *Enterodictyon Miill.-Arg. 

Paraphyses branched, interwoven above. 

Spores elongate, multi-septate, colourless 5. Chiodecton Ach. 

Spores elongate, multi-septate, brown 6. Sclerophyton Eschw. 

Spores muriform, colourless 7. *Minksia Miill.-Arg. 

Spores muriform, brown 8. *Enterostigma Miill.-Arg. 

Thallus with Phycopeltis gonidia (epiphyllous). 
Paraphyses free. 

Spores unequally 2-celled, colourless 9. *Pycnographa Miill.-Arg. 

Paraphyses branched, interwoven above. 

Spores elongate, multi-septate, colourless 10. *Mazosia Massal. 

XXI. DlRINACEAE 

A small family, which is associated with and often included under 
Graphidaceae. The thallus is crustaceous and corticate on the upper 
surface, the cortex being formed of palisade hyphae. Algal cells Trente- 
polilia. Apothecia are rounded or with a tendency to elongation, and, in 
addition to a thin proper margin, possess a stout thalline margin ; the 
hypothecium is thick and carbonaceous. There are two genera : Dirina 
with twelve species has a wide distribution ; Dirinastrum is monotypic and 
occurs on maritime rocks in Australia. In both the spores are elongate- 
septate, differing only in colour : 

Spores colourless I. Dirina Fr. 

Spores brown 2. *Dirinastrum Miill.-Arg. 

XXII. ROCCELLACEAE 

The Roccellaceae differ from the preceding Dirinaceae chiefly in the 
fruticose thallus which is more or less characteristic of all the genera, though 
in Roccellographa it expands into foliose dimensions and in Roccellina is 
reduced to short podetia-like processes from a crustose base. The fronds 
mostly long and strap-shaped are protected in most of the genera by 
a cortex of compact palisade hyphae; in a few the outer hyphae are parallel 
with the long axis. The medulla is of parallel hyphae, either loose or 
compact. The algal cells are Trentepohlia. 

The apothecia are lateral except in Roccellina where they occur at the 
tips of the short upright fronds, and only in Roccellaria is there no thalline 
margin. They are superficial in all of the genera except Roccellographa, in 
which they are immersed and almost closed, recalling the perithecia-like 



3 2 4 SYSTEMATIC 

fruits of Chiodecton (sect. Enterographa). The spores are elongate, narrow, 
pluri-septate, and colourless or brownish, except in Darbishirella in which 
they are ovoid, 2-septate and brown. 

The affinity of Dirinaceae and Roccellaceae with Graphidaceae was first 
indicated by Reinke 1 and elaborated later by Darbishire 2 in his monograph 
of Roccellaceae. The apothecia in some species of Dirina are ellipsoid rather 
than round ; in several genera of Roccellaceae they are distinctly lirellate, 
and in Roccella itself some species have ellipsoid fruits. The fruticose thallus 
is predominant in Roccellaceae, but its evolution from the crustaceous type 
may be traced through Roccellina which is partly crustaceous and only 
imperfectly fruticose. 

In most of the genera only one species is recorded. Roccella, represented 
by twelve species, is well known for its dyeing properties, and has a wide 
distribution. Like other Graphidineae they are mainly plants of warm 
regions, mariy of them exclusively maritime rock-dwellers. 

The following synopsis of the genera is the one given by Darbishire in 
his monograph. 

Cortex fastigate, of palisade hyphae. 
Spores colourless. 

Hypothecium black-carbonaceous. 
Apothecia round. 

Thallus fruticose I. Roccella DC. 

Thallus crustaceous-fruticose 2. *Roccellina Darbish. 

Apothecia lirellate 3. *Reinkella Darbish. 

Hypothecium colourless. 

Gonidia present under the hypothecium 4. *Pentagenella Darbish. 

Gonidia absent from hypothecium 5. *Combea De Not. 

Spores brown or brownish. 

Medulla of parallel somewhat loose hyphae 6. *Schizopelte Th. Fr. 

Medulla solid, black 7. *Simonyella Steiner. 

Cortex fibrous, of parallel hyphae. 
Apothecia round. 

Hypothecium black-carbonaceous. 

Apothecia with thalline margin ; . . . 8. *Dendrographa Darbish. 

Apothecia with proper margin 9. *Roccellaria Darbish. 

Hypothecium colourless 10. *Darbishirella A. Zahlbr. 

Apothecia lirellate II. *Ingaderia Darbish. 

SUBSERIES 3. CYCLOCARPINEAE 

This last subseries includes the remaining twenty-nine families of Asco- 
lichens. They are very varied both in the fungal and the algal symbionts. 
The fruit is more or less a discoid open apothecium. The gonidia belong to 
different genera of Myxophyceae and Chlorophyceae, but the most frequent 
are Protococcaceae. Families are based largely on thalline structure. 

1 Reinke 1895. 2 Darbishire 1898. 



FAMILIES AND GENERA 325 

XXIII. LECANACTIDACEAE 

By many systematists this family is included under Graphidineae on 
account of the fruit structure which in some of the forms is carbonaceous 
and almost lirellate, and also because the algal symbiont is Trentepohlia. 
The thallus is primitive, being thinly crustaceous and non-corticate ; the 
apothecium has a black carbonaceous hypothecium in two of the genera, 
Lecanactis and Schismatomma (Platygrapha)\ in the third genus, Melam- 
pydiwn, it is colourless. The latter is monotypic, and the spores become 
muriform. In the other genera they are elongate and multi-septate. 

Apothecia with prominent proper margin i. Lecanactis Eschw. 

Apothecia with thin proper margin 2. *Melampydium Miill.-Arg. 

Apothecia with thalline margin 3. Schismatomma Flot. 

XXIV. PlLOCARPACEAE 

A small family with but one genus, Pilocarpon. It is distinguished as 
one of the few epiphyllous genera of lichens associated with Protococcaceous 
gonidia and with a distribution extending far beyond the tropics. The best 
known species, P. leucoblepJiarum, encircles the base of pine-needles with 
a white felted crust, or inhabits coriaceous evergreen leaves. Another species 
lives on fern leaves. The fruit is a discoid apothecium with a dark carbona- 
ceous hypothecium and proper margin, and with a second thalline margin. 
The paraphyses are branched and interwoven above. 

Spores elongate, 3-septate, colourless i. Pilocarpon Wain. 

XXV. CHRYSOTRICHACEAE 

This family now, according to Hue 1 , includes two genera, Crocynia and 
Chrysothrix. In both there is a thallus of interlaced hyphae with Protococ- 
caceous algae scattered through it or in groups. The structure is thus 
homoiomerous, and Hue has suggested for it a new series, "Intertextae." 
The only British species, Crocynia lanuginosa, first placed by Nylander 2 in 
Amphiloma and later transferred by him to Leproloma*, has a soft crustaceous 
lobate thallus, furfuraceous on the surface; no fructification has been found. 
A West Indian species, C. gossypina, has discoid apothecia with a thalline 
margin. There is only one species of Chrysothrix, Ch. nolitangere, which 
forms small clumps or tufts on the spines of Cactus in Chili. The structure 
is somewhat similar to that of Crocynia. 

Spores colourless, simple i- Crocynia Nyl. 

Spores colourless, 2-3-septate 2. *Chrysothrix Mont. 

1 Hue 1909. 2 Nylander 1855. 3 Nylander 1883. 



326 SYSTEMATIC 

t 
XXVI. THELOTREMACEAE 

A tropical or subtropical family of which the leading characteristic is 
the deeply sunk disc of the apothecium : it has a proper hyphal margin, 
and, round that, an overarching thalline margin. The apothecia occur singly, 
or they are united in a kind of pseudostroma : in Tremotylium several grow 
together, while in Polystroma each new apothecium develops as an outgrowth 
from the thalline margin of the one already formed, so that an upright, 
r branching succession of fruits is built up. It is a very unusual type of lichen 
fructification, with one species, P. Ferdinandezii, found in Spain and in 
Guiana. 

The thallus in all the genera is crustaceous with an amorphous (decom- 
posed) cortex; or it is non-corticate. The algal cells are Trentepohlia except 
in Phyllophthalmaria, an epiphyllous genus associated with the alga Phyco- 
peltis. In Polystroma the alga is unknown. 

Only one genus is represented in the British Isles. 

Apothecia growing singly. 

Thallus with Trentepohlia gonidia. 

Paraphyses numerous, unbranched, free. 
Spores colourless. 

Spores elongate, 2- or multi-septate i. *Ocellularia Spreng. 

Spores muriform 2. Thelotrema Ach. 

Spores brown. 

Spores elongate, septate .3. *Phaeotrema Miill.-Arg. 

Spores muriform 4. *Leptotrema Mont. 

Paraphyses scanty, branched. 

Spores muriform, brown 5- *Gyrostomum Fr. 

Thallus with Phycopeltis gonidia 6. *Phyllophthalmaria A. Zahlbr. 

Apothecia in pseudostromata. 

Apothecia united in tubercles 7. *Tremotylium Nyl. 

Apothecia united 'by the margins 8. *Polystroma Clem. 

XXVII. DlPL OSCHIS TA CEA E 

Scarcely differing from the preceding family except in the gonidia which 
are Protococcaceous algae. The thallus is crustaceous and non-corticate. 
The apothecia have a double margin but the outer thalline margin is less 
overarching than in Thelotremaceae. The spores in the two genera are 
somewhat peculiar: in Conotrema they are exceedingly long and divided 
by parallel septa into thirty to forty small cells ; in Diploschistes ( Urceolaria) 
they are large, muriform and brown. Conotrema contains two corticolous 
species ; Diploschistes about thirty species mostly saxicolous. Both genera 
are represented in the British Isles. 

Spores elongate, multi-septate, colourless i. Conotrema Tuck. 

Spores muriform, brown 2. Diploschistes Norm. 



FAMILIES AND GENERA 327 

XXVIII. ECTOLECHIACEAE 

A family of tropical epiphyllous lichens that are associated with Proto- 
coccaceous gonidia. The thallus is primitive in character, mostly a weft of 
hyphae with intermingled algal cells, described as homoiomerous. 

The apothecia are without a thalline margin, and with a scarcely 
developed proper margin : their affinity is with the Lecideaceae, though in 
two genera, Lecaniella and ArtJiotJieliopsis, there are gonidia below the 
hypothecium, a character of Lecanoraceae. The genera are nearly all 
monotypic ; in Sporopodium has been included Lecidea phyllocJiaris YVainio 
(Sect. Gonotheciuni), which is distinguished by hymenial gonidia. 

Apothecia at first covered by a "veil." 

Spores elongate, colourless, septate i. *Asterothyrium Mull.-Arg. 

Apothecia uncovered from the first. 

Gonidia not present below the hypothecium. 
Paraphyses unbranched, free. 

Spores muriform 2. *Lopadiopsis Wain. 

Paraphyses branched. 

Spores i-septate 3. *Actinoplaca Mi.ill.-Arg. 

Spores elong'ate, multi-septate 4. *Tapellaria Miill.-Arg. 

Spores muriform 5. *Sporopodium Mont. 

(ionidia present below the hypothecium. 

Spores elongate, 2-septate 6. *Lec'aniella Wain. 

Spores muriform 7. *Arthotheliopsis Wain. 

XXIX. GYALECTACEAE 

The algal cells in this family are filamentous; either Myxophyceae 
(Scytoneina) or Chlorophyceae ( Trentepohlia or Phyllactidium). The thallus 
is crustaceous, and in some cases homoiomerous, as in Petractis, where the 
alga, Scytonema, penetrates the substratum as deeply as the hyphae. Mono- 
phiale, a tropical genus, possesses two kinds of gonidia : the species that 
grow on bark or mosses are associated with Trentepohlia ; others that have 
invaded the surface of leathery evergreen leaves resemble most epiphyllous 
lichens in being associated with the leaf alga Phyllactidium (Phycopeltis). 
Some species of Trentepohlia exhale when moist an odour of violets. This 
scent is retained in at least one genus, Jonaspis. 

The apothecia are superficial, and are soft, waxy and bright-coloured, 
with prominent margins which are however entirely hyphal : the affinity is 
therefore with Lecideaceae. In one genus, Sagiolechia, the fruit is carbona- 
ceous and dark coloured. The spores of all the genera are colourless. 

Apothecia waxy, bright-coloured. 
Thallus with Scytonema. gpnidia. 

Spores elongate, 3-septate i. Petractis Fr. 



328 SYSTEMATIC 

Thallus with Trentepholia gonidia. 
Asci 6-8-spored. 

Spores simple 2. Janaspis Th. Fr. 

Spores i-septate 3. *Microphiale A. Zahlbr. 

Spores septate or muriform 4. Gyalecta Ach. 

Asci i2-many-spored. 

Spores i-septate 5. *Ramonia Stizenb. 

Spores fusiform or acicular, many-septate ...6. Pachyphiale Lonnr. 
Apothecia carbonaceous. 

Spores elongate, 2-3-septate 7. *Sagiolechia Massal. 

XXX. COENOGONIA CEA E 

There are only two genera in this small family, Coenogonium with Trente- 
pohlia gonidia, and Racodium with Cladophora. Both genera follow the algal 
form and are filamentous. In Coenogonium the filaments are sometimes 
matted into a loose felted expansion. The genus is mainly tropical or 
subtropical and mostly rather light-coloured. There is only one British 
species, C. ebeneum 1 , a sterile form, in which the hyphae are very dark-brown ; 
it often covers large areas of stone or rock with its sooty-like creeping 
filaments. 

Racodium includes 2 (?) species. One of these, R. rupestre, is sterile and 
resembles C, ebeneum in form and colour. 

The apothecia of Coenogonium are waxy and light-coloured ; they are 
borne laterally on the filaments; the spores are simple or I -septate. 

Thallus with Trentepohlia gonidia i. Coenogonium Ehrenb. 

Thallus with Cladophora gonidia 2. Racodium Fr. 

XXXI. LECIDEACEAE 

One of the largest lichen families as regards both genera and species, 
and of world-wide distribution. The algal cells are Protococcaceae. The 
thallus is mostly crustaceous but it becomes squamulose in Psora, a section 
of Lecidea\ and in Sphaerophoropsis, a Brazilian genus, there are small 
upright fronds or stalks with lateral apothecia. The prevailing colour of 
the thallus is some shade of grey, but it ranges from white or yellow to 
dark-brown or almost black. Cephalodia appear in some of the species. 

The apothecia have a proper margin only, no gonidia taking part in the 
fruit-formation. They may be soft and waxy (biatorine) or hard and 
carbonaceous (lecideine). The genera are mainly based on spore characters 
which are very varied. 

The arrangement of genera given below follows that of Zahlbruckner ; 
in several instances, both as to the limitations of genera and to the nomen- 
clature, it differs from that of British text-books, though the general principle 
of classification is the same. 

1 Lorrain Smith 1906. 



FAMILIES AND GENERA 329 

Thallus crustaceous non-corticate. 
Spores simple. 

Spores small, thin-walled. 

Spores colourless i. Lecidea Ach. 

Spores brown 2. *Orphniospora Koerb. 

Spores large, thick-walled 3. Mycoblastus Norm. 

Spores i -septate. 

Spores small, thin-walled 4. Catillaria Th. Fr. 

Spores large, thick-walled 5. Megalospora Mey. and Flot. 

Spores elongate, 3-multi-septate. 

Spores elongate, narrow, thin-walled 6. Bacidia A. Zahlbr. 

Spores elongate, large and thick- walled 7. Bombyliospora De Not. 

Spores muriform. 

Spores colourless; on trees 8. Lopadium Koerb. 

Spores colourless to brown ; on rocks 9. Rhizocarpon Th. Fr. 

Thallus warted or squamulose, corticate. 

Spores elongate, i-y-septate, thin- walled 10. Toninia Th. Fr. 

Thallus of upright podetia-like small fronds. 

Spores ellipsoid, becoming I -septate n. *Sphaerophoropsis Wain. 

XXXII. PHYLLOPSORACEAE 

A small family of exotic lichens with a somewhat more developed thallus 
than that of the Lecideaceae, being in both of the genera squamulose or 
almost foliose. 

The apothecia are without a thalline margin ; they are biatorine or 
lecideine ; the hypothecium is formed of plectenchyma and is purple-red 
in one species, Phyllopsora furfuracea. The two genera differ only in spore 
characters. There are fifteen species, mostly corticolous, belonging to 
Pliyllopsora ; only one, from New Zealand, is recorded for Psorella. 

Spores simple i. *Phyllopsora Miill.-Arg. 

Spores elongate, septate 2. *Psorella Miill.-Arg. 

XXXIII. CLADONIACEAE 

Associated with Lecideaceae in the type of apothecium, but differing 
widely in thallus formation. The latter is of a twofold type : the primary 
thallus is crustaceous, squamulose, or very rarely foliose ; the secondary 
thallus or podetium, upright, simple or branched, is terminated by the 
apothecia, or broadens upwards to cup-like scyphi. Algal cells, Protococ- 
caceae, according to Chodat, Cystococcus. 

Much attention has been given to the origin and development of the 
podetia in this family. They are superficial on granule or squamule 
except in the monotypic Himalayan genus Gymnoderma where they are 
marginal on the large leaf-like lobes. Though in origin the podetia are 
doubtless fruit stalks, they have become in most cases vegetative in function. 



330 SYSTEMATIC 

The fruits are coloured yellowish, brown or red (or dark and carbonaceous 
in Pilophorus), and are borne on the tips of the branches or on the margins 
of the scyphi. In Glossodium and Thysanothecium the former from New 
Granada, the latter from Australia the apothecia occupy one side of the 
widened surface at the tips. 

Cephalodia are developed on the primary thallus of Pilophorus, and on 
the podetia of Stereocaulon and Argopsis. 

Podetia simple, short, not widening upwards. 
Podetial stalks naked. 

Primary thallus thin, continuous I. Gomphillus Nyl. 

Primary thallus granular or squamulose ... 2. Baeomyces Pers. 
Primary thallus foliose. 

Podetia superficial 3. *Heteromyces Miill.-Arg. 

Podetja marginal 4. *Gymnoderma 1 Nyl. 

Podetial stalks granular, squamulose 5. Pilophorus Th. Fr. 

Podetia short, widening upwards. 

Podetia simple above, rarely divided ... 6. *Glossodium Nyl. 

Podetia lobed, leaf-like 7. *Thysanothedum Berk. & Mont. 

Podetia elongate, variously branched, or scy-1 

' \ 8. Cladoma Hill, 
phous and hollow J 

Podetia elongate, not scyphous, the stalks solid. 

Spores elongate, septate 9. Stereocaulon Schreb. 

Spores muriform 10. *Argopsis Th. Fr. 

XXXIV. GYROPHORACEAE 

A small family of foliose lichens allied to Lecideaceae by the character 
of the fruit a superficial apothecium in the formation of which the gonidia 
take no share. There are only three genera, distinguished by differences in 
spore and other characters. Dermatiscum has light-coloured thallus and 
fruits ; of the two species, one occurs in Central Europe, the other in North 
America. Umbilicaria and GyropJiora are British; they are dark-coloured 
rock-lichens and are extremely abundant in Northern regions where they 
are known as "tripe de roche." Algal cells Protococcaceae. 

Umbilicaria, Dermatiscum, and some species of Gyrophora are attached 
to the substratum by a central point. Other species of Gyrophora are 
rhizinose. In all there is a cortex of plectenchyma above and below. In 
Gyrophora the thallus may be monophyllous as in Umbilicaria, or poly- 
phyllous and with or without rhizinae. New lobes frequently arise from 
protuberances or warts on the older parts of the thallus. At the periphery, 
in most species, growth is equal along the margins, in G. erosa 2 the edge is 
formed of numerous anastomosing lobes with lateral branching, the whole 
forming a broadly meshed open network. Further back the tissues become 
continuous owing to the active growth of the lower tissue or hypothallus, 

1 Neophyllis Wils. is synonymous with Gymnoderma. 2 Lindau 1 899. 



FAMILIES AND GENERA 331 

which grows out from all sides and meets across the opening. The overlying 
layers, with gonidia, follow more slowly, but they also in time become 
continuous, so that the "erose" character persists only near the periphery. 
This forward growth of the lower thallus occurs in other species, though to 
a much less marked degree. 

There is abundant detritus formation in this family; the outer layers of 
the cortex are continually being sloughed, the dead tissues lying on the 
upper surface as a dark gelatinous layer, continuous or in small patches. 
On the under surface the cast-off cortex gathers into a loose confused mass 
of dead tissues. 
Asci 8-spored. 

Spores mostly simple (disc gyrose) i. Gyrophora Ach. 

Spores i-septate 2. *Dermatiscum Nyl. 

Asci i-2-spored. 

Spores muriform 3. Umbilicaria Hoffrn. 

XXXV. ACAROSPORACEAE 

Thallus foliose, squamulose or crustaceous, sometimes scarcely developed. 
Algal cells Protococcaceae. 

Into this family Zahlbruckner has gathered the genera in which the 
asci are many-spored, as he considers that a character of great importance 
in determining relationship, but he has in doing so overlooked other very 
great differences. The fruit-bodies are round and completely enclosed in 
a thalline wall in Thelocarpon, which has however no perithecial wall. They 
have a proper margin only (lecideine) in Biatorella, and a thalline margin 
(lecanorine) in the remaining genera. In Acarospora the apothecia are sunk 
in the thallus. Stirton's genus Cryptothecia^ is allied to Tfielocarpon in the 
fruit-formation, but the basal thallus is well developed and the spores are 
few in number and variously divided. 

Thallus none. 

Apothecia (or perithecia) in thalline warts i. Thelocarpon Nyl. 

Thallus crustaceous. 

Apothecia lecideine ; spores simple 2. Biatorella Th. Fr. 

Apothecia lecanorine ; spores septate 3. *Maronea Massal. 

Thallus of small squamules 4. Acarospora Massal. 

Thallus almost foliose, attached centrally 5. *Glypholecia Nyl. 

XXXVI. EPHEBACEAE 

A family of very simple structure either filamentous, foliose or crustaceous. 
The algal cells which give a dark colour to the thallus are Stigonema or 
Scytonema, members of the blue-green Myxophyceae, and consist of minute 
simple or branched filaments single cell-rows in Scytonema, compound in 

Stigonema. 

1 Stirton 1877, p. 164. 



33 2 SYSTEMATIC 

In some of the genera the lichen hyphae travel within the gelatinous 
sheath of the filaments, both algae and hyphae increasing by apical growth 
so that filaments many times the length of the alga are formed as in 
Ephebe. In others the filaments scarcely increase beyond the normal size 
of the alga as in Thermutis (Gonionema); or the gelatinous algal cells may 
be distributed in a stratum of hyphae. 

The apothecia are minute and almost closed; they may be embedded 
in swellings of the thallus, or are more or less superficial. The spores are 
rather small, colourless and simple or I -septate. 

The lichens of this family are rock-dwellers and are mostly to be found 
in hilly or Alpine regions. A tropical species, Leptogidium dendriscum, occurs 
in sterile condition in south-west Ireland. There are few species in any of 
the 'genera. 

Algal cells Scytonema. 

Thallus minutely fruticose, non-corticate I. Thermutis Fr. 

Thallus minute, of felted filaments, cortex one) _ ,, 7 . 

> 2. *Leptodendnscum Wain, 
cell thick I 

Thallus of elongate filaments, cortex of several cells 3. Leptogidium Nyl. 

Thallus foliose or fruticose, cellular throughout 4. Polychidium Ach. 

Thallus crustaceous, non-corticate 5. Porocyphus Koerb. 

Algal cells Stigonema. 

Thallus minutely fruticose, non-corticate 6. Spilonema Born. 

Thallus of long branching .filaments. 

Spores septate ; paraphyses wanting 7. Ephebe Fr. 

Spores simple ; paraphyses present 8. Ephebeia Nyl. 

Thallus crustaceous; upper surface non-corticate,! . . . 

. \ 9. *Pterygtopsis Wain, 

lower surface corticate J 



XXXVII. P YRENOPSIDA CEA E 

In this family are included gelatinous lichens of which the gonidium is 
a blue-green alga with a thick gelatinous coat, either Gloeocapsa (including 
Xanthocapsd) or Chroococcus. In Gloeocapsa and Chroococcus the gelatinous 
envelope is often red, in Xanthocapsa it is yellow, and these colours persist 
more or less in the lichens, especially in the outer layers. 

The thallus is in many cases a formless gelatinous crust of hyphal 
filaments mingling with colonies of algal cells as in Pyrenopsis; but small 
fruticose tufts are characteristic of Synalissa, and larger foliose and fruticose 
thalli appear in some exotic genera. A plectenchymatous cortex is formed 
on the thallus of Forssellia, a crustaceous genus from Central Europe, with 
two species only; the whole thallus is built up of a kind of plectenchyma 
in some others, but in most of the genera there is no tissue formed. 

The apothecia, as in Ephebaceae, are generally half-closed. 



FAMILIES AND GENERA 333 

Thallus with Gloeocapsa gonidia. 
Thallus crustaceous. 

Spores simple i. Pyrenopsis Nyl. 

Spores i-septate 2. -Cryptothele Forss. 

Thallus shortly fruticose 3. Synalissa Fr. 

Thallus lobate, centrally attached 4. *Phylliscidium Forss. 

Thallus with Chroococcus gonidia. 

Thallus crustaceous 5. Pyrenopsidium Forss. 

Thallus lobate, centrally attached 6. *Phylliscum Nyl. 

Thallus with Xanthocapsa gonidia. 
Thallus crustaceous. 
Thallus non-corticate. 
Spores simple. 

Apothecia open, asci 8-spored 7. Psorotichia Forss. 

Apothecia covered, asci many-spored 8. *Gonohymenia Stein. 

Spores i -septate. 

Apothecia closed 9. *Collemopsidium Nyl. 

Thallus with plectenchymatous cortex 10. *Forssellia A. Zahlbr. 

Thallus lobate, centrally attached. 
Spores simple. 

Thallus plectenchymatous throughout u. *Anema Nyl. 

Thalline tissue of loose hyphae 12. *Thyrea Massal. 

Cortex of upright parallel hyphae 13. *Jenmania Wacht. 

Spores i -septate. 

Thalline tissue of loose hyphae 14. *Paulia Fe"e. 

Thallus fruticose. 

Thallus without a cortex 15. *Peccania Forss. 

Thallus with cortex of parallel hyphae 16. *Phloeopeccania Stein. 

XXXVIII. LlCHlNACEAE 

The only family of lichens associated with Rivularia gonidia, the 
trichomes of which retain their filamentous form to some extent in the 
more highly developed genera; they lie parallel to the long axis of the 
squamule or of the frond except in LicJiinella in which genus they are 
vertical to the surface. The thallus may be crustaceous, or minutely foliose, 
or fruticose; in all cases it is dark-brown in colour, and the gelatinous 
character is evident in the moist condition. The best known British genus 
is Licliina which grows on rocks by the sea. 

The apothecia are more or less immersed in the tissue; in Pterygium and 
Steinera they are open and superficial (the latter monotypic genus confined 
to Kerguelen). They are also open in Lichinella and Homopsella^ both very 
rare genera. The spores are colourless and simple except in Pterygium arid 
Steinera where they are elongate, and i-3-septate. 
Thallus crustaceous squamulose. 

Apothecia immersed in thalline warts i. *Calothricopsis Wain. 

Apothecia superficial, with thalline margin 2. *Steinera A. Zahlbr. 

Apothecia superficial, without a thalline margin 3. Pterygium Nyl. 



334 SYSTEMATIC 

Thallus of small fruticose fronds. 

Gonidia occupying the central strand 4. *Lichinodium Nyl. 

Gonidia not in the centre. 

Apothecia immersed 5. Lichina Ag. 

Apothecia superficial. 

Paraphyses present 6. *Lichinella Nyl. 

Paraphyses absent 7. *Homopsella Nyl. 



XXXIX. CoLLEMACEAE 

The most important family of the gelatinous lichens and the most 
numerous. Collema is historically interesting as having first suggested the 
composite thallus. Algal cells, Nostoc, which retain the chain-like form 
except in Leprocollema, a doubtful member of the family. The thallus varies 
from indeterminate crusts to lobes of considerable size ; occasionally the 
lobes are narrow and erect, forming minute fruticose structures. In the 
more primitive genera the thallus is non-corticate, but in the more evolved, 
the apical cells of the hyphae coalesce to form a continuous cellular cortex, 
one or more cells thick, well marked in some species, in others rudimentary; 
the formation of plectenchyma also occurs occasionally in the apothecial 
tissues of some non-corticate species. 

The apothecia are superficial except in Pyrenocollema, a monotypic genus 
of unknown locality. They are generally lecanorine, with gonidia entering 
into the formation of the apothecium : in some genera they are lecideine or 
biatorine, being formed of hyphae alone. The spores are colourless and vary 
in form, size and septation. 

Apothecia immersed ; spores fusiform, i-septate i. *Pyrenocollema Reinke. 

Apothecia superficial. 
Thallus without a cortex. 

Spores simple, globose or ellipsoid. 

Thallus crustaceous 2. *Leprocollema Wain. 

Thallus largely squamulose-fruticose. 

Apothecia lecideine (dark-coloured) 3. *Leciophysma Th. Fr. 

Apothecia lecanorine 4. Physma Massal. 

Spores variously septate or muriform. 

Apothecia biatorine (light-coloured) 5. *Homothecium Mont. 

Apothecia lecanorine 6. Collema Wigg. 

Thallus with cortex of plectenchyma. 
Spores simple. 

Spores globose 7. Lemmopsis A. Zahlbr. 

Spores ellipsoid, with thick subverrucose wall... 8. *Dichodium Nyl. 

Spores vermiform, spirally curved 9. *Koerberia Massal. 

Spores variously septate or muriform. 

Apothecia biatorine (light-coloured) 10. *Arctomia Th. Fr. 

Apothecia lecanorine u. Leptogium S. F. Gray. 



FAMILIES AND GENERA 335 

XL. HEPPIACEAE 

A family belonging to the "blue-green" series as it is associated with 
a gelatinous alga, Scytonema, but is of almost entirely cellular structure and 
is non-gelatinous. The thallus is squamulose or minutely foliose, or is formed 
of narrow almost fruticose lobes; the apothecia are semi-immersed; the asci 
are 4-many-spored. 

Heppia is a wide-spread genus both in northern and tropical regions 
with about forty species that live on soil or rock. So far, no representative 
has been recorded in our Islands. 

Spores simple, colourless, globose or ellipsoid i. *Heppia Naeg. 

Spores muriform, colourless, ellipsoid 2. *Amphidium 1 Nyl. 

X L I . PA N\A RIACEAE 

The members of this family are also non-gelatinous, though for the most 
part associated with blue-green gelatinous algae, Nostoc or Scytonema. The 
gonidia are bright-green in the genera Psoroma and Psoromaria, the former 
often included under Lecanora, but too closely resembling Pannaria to be 
dissociated from that genus. 

The -thallus varies from being crustaceous to squamulose or foliose, and 
has a cortex of plectenchyma on the upper and sometimes also on the 
lower surface. The apothecia are superficial or lateral and with or without 
a thalline margin (lecanorine or biatorine), the spores are colourless. 

Zahlbruckner has included Hydrotkyria in this family. It is a monotypic 
aquatic genus found in North America and very closely allied to Peltigera. 
The British species of the genus, familiarly known as Coccocarpia, have 
been placed under Parmeliella, the former name being restricted to the 
tropical or subtropical species first assigned to Coccocarpia and distinguished 
by the cortex, the hyphae forming it lying parallel with the surface though 
forming a regular plectenchyma. 

An Antarctic lichen TJielidea corrugata with Palmetto, gonidia is doubt- 
fully included: the thallus is foliose, the apothecia biatorine with colourless 
i -septate spores. 

Thallus with bright-green gonidia. 

With Palmetto, i. *Thelidea Hue. 

With Protococcaceae. 

Apothecia non-marginate (biatorine) 2. *Psoromaria Nyl. 

Apothecia marginate 3. Psoroma Nyl. 

Thallus with Scytonema gonidia. 

Apothecia marginate, spores i-septate 4. Massalongia Koerb. 

Apothecia non-marginate ; spores simple. 

Upper surface smooth 5. *Coccocarpia Pers. 

Upper surface felted 6. *Erioderma Fe"e. 

1 A. Zalilbruckner, in Oesterr. hot. Zeitschr. 1919, p- 163. 



336 SYSTEMATIC 

Thallus with Nostoc gonidia. 

Apothecia marginate ; spores simple 7. Pannaria Del. 

Apothecia non-marginate ; spores various. 

Thallus crustaceous or minutely squamulose ... 8. Placynthium Ach. 

Thallus squamulose, cortex indistinct 9. *Lepidocollema Wain. 

Thallus squamulose or foliose, cortex cellular ...10. Parmeliella Miill.-Arg. 

Thallus foliose, thin veined below 11. *Hydrothyria Russ. 

XLII. STICTACEAE 

Thallus foliose, mostly horizontal, with a plectenchymatous cortex on 
both surfaces, a tomentum of hair-like hyphae taking the place of rhizinae 
on the lower surface. Algal cells Protococcaceae or Nostoc. Cephalodia and 
cyphellae or pseudocyphellae often present. Apothecia superficial or lateral ; 
spores colourless or brown, variously septate. 

The highly organized cortex and the presence of aeration organs 
cyphellae or pseudocyphellae which are almost solely confined to the 
genus Sticta give this family a high position as regards vegetative develop- 
ment. The two genera are of wide distribution, but Sticta is more abundant 
in the Southern Hemisphere. Lobaria pulmonaria is one of our largest 
lichens. 

Under surface dotted with cyphellae or pseudo-} 



, I. Sticta Schreb. 
cyphellae ... 

Under surface without these organs 2. Lobaria Schreb. 

XLII I. PELTIGERACEAE 

A family of heteromerous foliose lichens containing in some instances 
blue-green (Nostoc), in others bright-green (Protococcaceae) gonidia, and 
thus representing a transition between these two series. They have large 
or small lobes and grow on the ground or on trees. 

Cephalodia, either ectotrophic (Peltidea) or endotrophic (Solorina), occur 
in the family and further exemplify the capacity of the fungus hyphae to 
combine with different types of algae. 

The upper surface is a wide cortex of plectenchyma, which in some 
forms (Nephromium) is continued below. In the non-corticate under surface 
of Peltigera, the lower hyphae grow out in hairs or rhizinae, very frequently 
brown in colour. Intercalary growth of the upper tissues stretches the 
thallus and tears apart the lower under surface so that the hair-bearing 
areas become a network of veins, with the white exposed medulla between. 
In Peltigera canina there is further growth and branching of the hyphae in 
the veins, adding to the bulk of the interlacing ridges. 

From all other foliose lichens Peltigeraceae are distinguished by the 
flat wholly appressed or peltate apothecia without a thalline margin which 
arise mostly on the upper surface, but in Nephromium on the extreme 



FAMILIES AND GENERA 337 

margin of the under surface, the tip of the fertile lobe in that case is turned 
back as the apothecium matures, so that the fruit eventually faces the light. 
In Nephroma has been included Eunephroma with bright-green gonidia and 
Nephromium with blue-green. 

Bitter 1 has recorded the finding of apothecia on the under surface of 
Peltigera malacea and not at the margin, as in Nephromium. The plant was 
otherwise normal and healthy. Solorinella, from Central Europe and 
Asteristion from Ceylon are monotypic genera with poorly developed thalli. 

Thallus poorly developed. 

Asci 6-8-spored; spores 3-5 -septate i. *Asteristion Leight. 

Asci many-spored ; spores i-septate 2. *Solorinella Anzi. 

Thallus generally well developed. 

Apothecia superficial, sunk in the thallus 3. -Solorina Ach. 

Apothecia terminal on upper surface of lobes 4. Peltigera Willd. 

Apothecia terminal on lower surface of lobes 5. Nephroma Ach. 

XLIV. PERTUSARIACEAE 

Thallus crustaceous, often rather thick and with an amorphous cortex 
on the upper surface. Algal cells Protococcaceae. Apothecia solitary or 
several immersed in thalline warts, generally with a narrow opening which 
barely exposes the disc, and which in one genus, Perforaria, is so small as 
almost to constitute a perithecium ; spores are often very large and with 
thick walls; some if not all are multinucleate and germinate at many points. 

In the form of the fruit, this family stands between Pyrenocarpeae and 
Gymnocarpeae, though more akin to the latter. Perforaria, with two species, 
belongs to New Zealand and Japan. Pertusaria has a world-wide distri- 
bution, and Varicellaria, a monotypic genus, with a very large two-celled 
spore, is an Alpine plant, recorded from Europe and from Antarctic 
America. 

Spores simple. 

Apothecia with pore-like opening I. *Perforaria Miill.-Arg. 

Apothecia with a wider opening 2. Pertusaria DC. 

Spores i-septate 3. Varicellaria Nyl. 

XLV. LECANORACEAE 

Thallus mostly crustaceous, occasionally squamulose or very rarely 
minutely fruticulose. The squamulose thallus is corticate above, the under 
surface appressed and attached to the substratum by penetrating hyphae, 
often effigurateat the circumference. Algal cells Protococcaceae. Apothecia 
well distinguished by the thalline margin; spores colourless, simple or 
variously septate or muriform. 

1 Bitter 1904*. 



33 8 SYSTEMATIC 

Lecanora, Ochrolechia, Lecania, Haematomma and Phlyctis are cosmo- 
politan genera, some of them with a very large number of species; the other 
genera are more restricted in distribution and generally with few species. 

The genus Candelariella is of uncertain position; the spores are 8 or 
many in the ascus and are simple or I -septate, and not unfrequently become 
polarilocular as in Caloplacaceae, but there is no parietin present. 
Algae distributed through the thallus. Spores simple i. *Harpidium Koerb. 
Algae restricted to a definite zone. 
Spores simple. 

Thallus grey, white or yellowish. 

Spores rather small \2. Lecanora Ach. 

Spores large 3. Ochrolechia Massal. 

Thallus bright yellow. 

Spores simple or I -septate 4. Candelariella Miill.-Arg. 

Spores i-septate (rarely pluri-septate). 
Paraphyses free. 

Thallus squamulose, effigurate 5. Placolecania Zahlbr. 

Thallus crustaceous. 

Apothecial disc brownish 6. Lecania Zahlbr. 

Apothecial disc flesh-coloured 7. Icmadophila Trevis. 

Paraphyses branched, intricate 8. *Calenia Miill.-Arg. 

Spores elongate, pluri-septate. 

Apothecia superficial 9. Haematomma Massal. 

Apothecia immersed. 

Paraphyses free 10. *Phlyctella Miill.-Arg. 

Paraphyses branched, intricate 11. *Phlyctidia Miill-Arg. 

Spores muriform. 

Apothecia superficial 12. *Myxodictyon Massal. 

Apothecia immersed 13. Phlyctis Wallr. 

XLVI. PARMELIACEAE 

A very familiar family of foliose lichens. Genera and species are dorsi- 
ventral and stratose in structure, though some Cetrariae are fruticose in 
habit. Algal cells are Protococcaceae; in Physcidia they are Palmellae, In 
every case the upper surface of the thallus is corticate and generally of 
plectenchyma, the lower being somewhat .similar, but in Heterodea and 
Physcidia, monotypic Australasian genera, the upper cortex is of branching 
hyphae parallel with the surface, the lower surface being non-corticate. 

The Parmeliae are mostly provided with abundant rhizinae; in Cetrariae 
and Nephromopsis these are very sparingly present, while in Anzia (including 
Pannop annelid) the medulla passes into a wide net-like structure of anasto- 
mosing hyphae. 

In Heterodea, cyphellae occur on the under surface as in Stictaceae; and 
in Cetraria islandica bare patches have been described as pseudocyphellae. 
The latter lichen is one of the few that are of value as human food. Special 
aeration structures are present on the upper cortex of Parmelia aspidota. 



FAMILIES AND GENERA 339 

Thallus non-corticate below. 

Apothecia terminal I. *Heterodea Nyl. 

Apothecia superficial 2. *Physcidia Tuck. 

Thallus spongy below 3. *Anzia Stizenb. 

Thallus corticate below. 

Asci poly-spored 4. Candelaria Massal. 

Asci 8-spored. 

Spermatia acrogenous 5. Parmeliopsis Nyl. 

Spermatia pleurogenous. 

Apothecia superficial 6. Parmelia Ach. 

Apothecia lateral. 

Apothecia on upper surface 7. Cetraria Ach. 

Apothecia on lower surface 8. *Nephromopsis Miill.-Arg. 

XLVII. USNEACEAE 

This also is a familiar family of lichens, Usnea barbata the "bearded moss" 
being one of the first lichens noted and chronicled. Algal cells Protococ- 
caceae. Structure radiate, the upright or pendulous habit characteristic of 
the family securing all-round illumination. Special adaptations of the cortex 
or of the internal tissues have been evolved to strengthen the thallus against 
the strains incidental to their habit of growth as they are attached in 
nearly all cases by one point only, by a special sheath, or by penetrating 
hold-fasts. 

Apothecia are superficial or marginal and sometimes shortly stalked ; 
spores are simple or variously septate. 

Ranialina and Usnea, the most numerous, are cosmopolitan genera; 
Alectoria inhabits northern or hilly regions. 

The genus Evernia, also cosmopolitan, represents a transition between 
foliose and fruticose types; the fronds of the two species, though strap- 
shaped and generally upright, are dorsiventral and stratose, the gonidia 
for the most part lying beneath one surface; the other (lower) surface is 
either white or very dark-coloured. Everniopsis, formed of thin branching 
strap-shaped fronds, is also dorsiventral. 

A number of genera, TJiamnolia, Siphu/a, etc. are of podetia-like structure, 
generally growing in swards. Several of them have been classified with 
Cladoniae, but they lack the double thallus. One of these, Endocena, a 
sterile monotypic Patagonian lichen, with stiff hollow coralloid fronds, was 
classified by Hue 1 along with SipJiula\ recently he has transferred it to his 
family Polycaulionaceae 2 based on Polycauliona regale (Placodium frustu- ^ 
losnin Darbish.), and allied to Placodium Sect. T/iamnoma 3 . In recent studies 
Hue has laid most stress on thalline characters. He places the new family 
between "Ramalinaceae" and " Alectoriaceae." Dactylinaarctica is a common 
Arctic soil-lichen. 

1 Hue 1892. 2 Hue 1914. 3 Tuckerman 1872, p. 107. 

22 2 



340 SYSTEMATIC 

Thallus strap- shaped. 
Structure dorsiventraL 

Greyish-green above I. Evernia Ach. 

Whitish-yellow above 2. *Evemiopsis NyL 

Structure radiate alike on both surfaces. 

Fronds grey; medulla of loose hyphae 3. Ramalina Ach. 

Fronds yellow ; medulla traversed by strands 4. *Letharia A. Zahlbr. 

Thallus filamentous. 

Medulla a strong "chondroid" strand 5. Usnea DilL 

Medulla of loose hyphae. 

Spores simple 6. Alectoria Ach. 

Spores muriform, brown 7. *Oropogon Fr. 

Thallus of upright podetia-like fronds. 

Fronds rather long (about two inches), tapering,^ 8. Thamnolia Ach. (Cerania 

white I S. F. Gray). 

Fronds shorter, blunt 

Medulla solid 9. *Siphula Fr. 

Medulla partly or entirely hollow. 

Fronds swollen and tall (about two inches) 10. *Dactylina NyL 

Fronds coralloid, entangled n. *Endocena Cromb. 

Fronds short, upright 12. *Dufourea NyL 



XLVIII. CALOPLACACEAE 

In this family Zahlbruckner has included the squamulose or crustaceous 
lichens with colourless polarilocular spores, relegating those with more 
highly developed thallus or with brown spores to other families. He has 
also substituted the name Caloplaca for the older Placodium, the latter being, 
as he considers, less well defined. 

Algal cells are Protococcaceae. The thallus is mostly light-coloured, 
generally some shade of yellow, and, with few exceptions, contains parietin, 
which gives a purple colour on the application of potash. The squamulose 
forms are closely appressed to the substratum, and have often a definite 
rounded outline (effigurate). The spores have a thick median septum with 
a loculus at each end and a connecting canal 1 . 

In Blastenia the outer thalline margin is obscure or absent though 
gonidia are frequently present below the hymenium. Caloplacaceae occur 
all over the globe: they are among the most brilliantly coloured of all 
lichens. Polycauliona Hue 1 possibly belongs here: though based on thalline 
rather than on spore characters, one species at least has polarilocular spores. 

Apothecia with a distinct thalline margin i. Caloplaca Th. Fr. 

Apothecia without a thalline margin 2. Blastenia Th. Fr. 

1 See p. 188. Hue 1908. 



FAMILIES AND GENERA 341 

XLIX. TELQSCHISTACEAE 

PolariJocular colourless spores are the distinguishing feature of this 
family as of the Caloplacaceae. Algal cells Protococcaceae. The thallus 
of Teloschistaceae is more highly developed, being either foliose or fruticose, 
though never attaining to very large dimensions. The cortex of Xanthoria 
( foliose) is plectenchymatous, that of Teloschistes (fruticose) is fibrous. The 
species of both genera are yellow- or greenish-yellow due to the presence of 
the lichen-acid parietiru 

Both genera have a wide distribution over the globe, more especially in 
maritime regions. 

Thallus foliose i. Xanthoria Th. Fr. 

Thallus fruticose 2. Teloschistes Norm. 

L. B CELL! ACE AE 

A family of crustaceous lichens distinguished by the brown two-celled 
spores. Algal cells Protococcaceae. Zahlbruckner has included here Bufllia 
and Rinodina; the former with a distinctly lecideine fruit and with thinly 
septate spores; the latter lecanorine and with spores of the polarilocular 
type, with a very wide central septum pierced in most of the species by 
a canal which may or may not traverse the middle lamella of the wall. 
Rinodina is closely allied to Physciaceae, while Buellia has more affinity 
with Lecideaceae and is near to Rhizocarpon. 

Both genera are of world-wide distribution. 

Apothecia lecideine, without a thalline margin i. Buellia De Xot. 

Apothecia lecanorine, with a thalline margin 2. Rinodina MassaL 

LI. PHYSCIACEAE 

Thallus foliose or partly fruticose, and generally attached by rhizinae. 
Algal cells Protococcaceae. The spores resemble those of Rinodina, dark- 
coloured with a thick septum and reduced cell-lumina. As in that species 
there may be a second septum in each cell, giving a 3-septate spore; but 
that is rare. 

Pyxine, a tropical or subtropical genus, is lecanorine only in the very 
early stages; it soon loses the thalline margin. Anaptychia is differentiated 
from Physria by the subfruticose habit though the species are nearly all 
dorsiventral in structure, only a few of them being truly radiate and corticate 
on both surfaces. The upper cortex of Anaptychia is fibrous, but that 
character appears also in most species of Physcia either on the upper or the 
lower side. Physcia and Anaptychia are widely distributed. 

Thalline margin absent in apothecia I. *Pyxine XyL 

Thalline margin present in apothecia. 

Thallus foliose 2. Physcia Schreb. 

Thallus fruticose 3- Anaptychia Koerb. 



342 SYSTEMATIC 

C. *HYMENOLICHENS 

Fungus a Basidiomycete, akin to Thelephora. Algal cells Scytonema or 
Chroococcus. Thallus crustaceous, squamulose or foliose. Spores colourless, 
produced on basidia, on the under surface of the free thallus. 

The Hymenolichens 1 are few in number and are endemic in tropical 
or warm countries. They inhabit soil or trees. 

Thallus of extended lobes. 

Gonidia near the upper surface i. *Dictyonema Zahlbr. 

Gonidia in centre of tissue 2. *Cora Fr. 

Thallus squamulose, irregular 3. *Corella Wain. 

II. NUMBER AND DISTRIBUTION OF LICHENS 
i. ESTIMATES OF NUMBER 

Calculations have been made and published, once and again, as to the 
number of lichen species occurring over the globe or in definite areas. In 
1898 Fiinfstuck stated that about 20,000 different species had been described, 
but as many of them had been proved to be synonyms, and since many 
must rank as forms or varieties, the number of well-authenticated species 
did not then, according to his estimate, exceed 4000. Many additional 
genera and species have, however, been discovered since then. In Engler 
and Prantl's Pflanzenfamilien, over 50 families and nearly 300 genera find 
a place, but even in these larger groupings opinions differ as to the limits 
both of genera and families, and lichenologists would not all accept the 
arrangement given in that volume. 

Fiinfstuck has reckoned that of his estimated 4000, about 1500 are 
European and of these at least 1200 occur in Germany. Probably this is 
too low an estimate for that large country. Leighton in 1879 listed, in his 
British Lichen Flora, 1710 in all, and, as the compilation includes varieties, 
it cannot be considered as very far astray. On comparing it with Olivier's 2 
recent statistics of lichens, we find that of the larger fruticose and foliose 
species, 310 are recognized by him for the whole of Europe, 206 of these 
occurring in the British Isles. Leighton's estimate of similar species is 
about 145, without including varieties now reckoned as good species. In 
a more circumscribed area, Th. Fries 3 described for Spitzbergen about 210 
different lichens, a number that closely approximates to the 206 recent re- 
cords by Darbishire 4 for the same area. 

A general idea of the comparative numbers of the different types of 
lichens may be gathered from Hue's compilation of exotic lichens 5 , examined 

1 See p. 152. 2 Olivier 1907. 3 Th. Fries 1867. 

4 Darbishire 1909. 5 Hue 1892. 



NUMBER OF LICHENS 343 

or described by Nylander, and now in the Paris herbarium. There are 135 
genera with 3686 species. Of these, about 829 belong to the larger foliose 
and fruticose lichens (including Cladoniae)\ the remaining 2857 belong to 
the smaller kinds, most of them crustaceous. 



2. GEOGRAPHICAL DISTRIBUTION 

A. GENERAL SURVEY 

The larger foliose and fruticose lichens are now fairly well known and 
described for Europe, and the knowledge of lichens in other continents is 
gradually increasing. It is the smaller crustaceous forms that baffle the in- 
vestigator. The distribution of all lichens over the surface of the earth is 
controlled by two principal factors, climate and substratum ; for although 
lichens as a rule require only support, they are most of them restricted to one 
or another particular substratum, either organic or inorganic. As organisms 
which develop slowly, they require an unchanging substratum, and as sun- 
plants they avoid deeply shaded woodlands: their occurrence thus depends 
to a large extent on the configuration and general vegetation of the country. 

Though so numerous and so widely distributed, lichens have not evolved 
that great variety of families and genera characteristic of the allied fungi 
and algae. They conform to a few leading types of structure, and thus the 
Orders and Families are comparatively few, and more or less universal. 
They are most of them undoubtedly very old plants and were probably 
wide-spread before continents and climates had attained their present 
stability. Arnold 1 indeed considers that a large part of the present-day 
lichens were almost certainly already evolved at the end of the Tertiary 
period, and that they originated in a warm or probably subtropical climate. 
As proof of this he cites such genera as Graphis, Thelotrenia and Arthonia* 
which are numerous in the tropics though rare in the colder European 
countries ; and he sees further proof in the fact that many fruticose and 
gelatinous lichens do not occur further north than the forest belt, though 
they are adapted to cold conditions. Several genera that are abundant in 
the tropics are represented outside these regions by only one or few species, 
as for instance Conotrenia urceolatum and Bonibyliospora incana. 

During the Ice age of the Quaternary period, not many new species can 
have arisen, and such forms as were not killed off must have been driven 
towards the south. As the ice retreated the valleys were again stocked with 
southern forms, and northern species were left behind on mountain tops all 
over the globe. 

1 Arnold 1890. 

2 These genera are associated with Trentepohlia algae which are numerous and abundant in 
tropical climates, and their presence there may possibly account for these particular lichens. 



344 SYSTEMATIC 

In examining therefore the distribution of lichens, it will be found that 
the distinction between different countries is relative, certain families being 
more or less abundant in some regions than others, but, in general, nearly 
all being represented. Certain species are universal, where similar conditions 
prevail. This is especially true of those species adapted to extreme cold, as 
that condition, normal in polar regions, recurs even on the equator if the 
mountains reach the limit of perpetual snow ; the vertical distribution thus 
follows on the lines of the horizontal. 

In all the temperate countries we find practically the same families, with 
some few exceptions; there is naturally more diversity of genera and species. 
Genera that are limited in locality consist, as a rule, of one or few species. 
In this category, however, are not included the tropical families or genera 
which may be very rich in species: these are adapted to extreme conditions 
of heat and often of moisture, and cannot exist outside tropical or subtropical 
regions, extreme heat being more restricted as to geographical position than 
extreme cold. 

In the study of distribution the question which arises as to the place of 
origin of such widely distributed plants is one that is difficult to solve. 
Wainio 1 has attempted the task in regard to Cladonia, one of the most 
unstable genera, the variations of form, which are dependent on external 
circumstances, being numerous and often bewildering. In his fine mono- 
graph of the genus, 132 species are described and 25 of these are cosmo- 
politan. 

The distribution of Phanerogams is connected, as Wainio points out, 
with causes anterior to the present geological era, but this cannot be the 
case in a genus so labile and probably so recent as Cladonia, though some 
of the species have existed long enough to spread and establish themselves 
from pole to pole. Endemic species, or those that are confined to a com- 
paratively limited area, are easily traced to their place of origin, that being 
generally the locality where they are found in most abundance, and as 
a general rule in the centre of that area, though there may be exceptions: 
a plant for instance that originated on a mountain would migrate only in 
one direction towards the regions of greater cold. 

The difficulty of determining the primitive. stations of cosmopolitan, or 
of widely spread, species is much greater, but generally they also may be 
referred to their area of greatest abundance. Thus a species may occur 
frequently in one continent and but rarely in another, even where the con- 
ditions of climate, etc., are largely comparable. It may therefore be inferred 
that the plant has not yet reached the full extent of possible distribution in 
the less frequented area. As examples of this, Wainio cites, among other 
instances, Cladonia papillaria, which has a very wide distribution in Europe, 

1 Wainio 1897. 



DISTRIBUTION 345 

but, as yet, has been found only in the eastern parts of North America; and 
Cl. pycnodada, a plant which braves the climate of Cape Horn and the 
Falkland Islands, but has not travelled northward beyond temperate North 
America: the southern origin of that species is thus plainly indicated. Wainio 
also finds that evidence of the primitive locality of a very widely spread 
species may be obtained by observing the locality of species derived from 
it, which are as yet of limited distribution ; presumably these arose in the 
ancestral place of origin, though this indication is not always to be relied 
on. If, however, the ancestral plant has given rise to several of these rarer 
related species, those of them that are most closely allied to the primitive 
plant would be found near to it in the original locality. 

A detailed account of species distribution according to these indications 
is given by Wainio and is full of interest. No such attempt has been made 
to deal with any other group, and the distribution of genera and species can 
only be suggested. An exhaustive comparison of the lichens of different 
regions is beyond the purpose of our study and is indeed impossible as, 
except in some limited areas, or for certain species, the occurrence and dis- 
tribution are not fully known. It is in any case only tentatively that genera 
or species can be described as local or rare, until diligent search has been 
made for them over a wider field. The study of lichens from a floristic point 
of view lags behind that of most other groups of plants. The larger lichen 
forms have received more attention,' as they are more evident and more 
easily collected ; but the more minute species are not easily detected, and, 
as they are largely inseparable from their substratum of rocks, or trees, etc., 
on which they grow, they are often difficult to collect. They are also in 
many instances so indefinite, or so alike in outward form, that they are 
liable to be overlooked, only a m'icroscopic examination revealing the differ- 
ences in fruit and vegetative structure. 

Though much remains to be done, still enough is known to make the 
geographical distribution of lichens a subject of extreme interest. It will be 
found most instructive to follow the usual lines of treatment, which give the 
three great divisions : the Polar, the Temperate and the Tropical regions 
of the globe. 



B. LICHENS OF POLAR REGIONS 

Strictly speaking, this section should include only lichens growing within 
the Polar Circles; but in practice the lichens of the whole of Greenland and 
those of Iceland are included in the Arctic series, as are those of Alaska: 
the latitudinal line of demarcation is not closely adhered to. With the 
northern lichens may also be considered those of the Antarctic continent, 
as well as those of the islands just outside the Antarctic Circle, the South 



34 6 SYSTEMATIC 

Shetlands, South Orkneys, Tierra del Fuego, South Georgia and the Falkland 
Islands. During the Glacial period, the polar forms must have spread with 
the advancing cold ; as the snow and ice retreated, these forms have been 
left, as already stated, on the higher colder grounds, and representatives of 
polar species are thus to be found very far from their original haunts. There 
are few exclusively boreal genera: the same types occur at the Poles as in 
the higher subtemperate zones. One of the most definitely polar species, 
for instance, Usnea (Neuropogon] melaxantha grows in the whole Arctic zone, 
and, in the Antarctic, is more luxuriant than any other lichen, but it has also 
been recorded from the Andes in Chili, Bolivia and Peru, and from New 
Zealand (South Island). 

Cold winds are a great feature of both poles, and the lichens that by 
structure or habit can withstand these are the most numerous ; those that 
have a stout cortical layer are able to resist the low temperatures, or those 
that grow in tufts and thus secure mutual protection. In Arctic and Subarctic 
regions, 495 lichens have been recorded, most of them crustaceous. Among 
the larger forms the most frequently met are certain species of Peltigera, 
P armelia, Gyrophora, Cetraria, Cladonia, Stereocaulon and Alectoria. Among 
smaller species Lecanora tartarea spreads everywhere, especially over other 
vegetation, Lecanora varia reaches the farthest limits to which wood, on 
which it grows, has drifted, and several species of Placodium occur con- 
stantly, though not in such great abundance." Over the rocks spread also 
many crustaceous Lecideaceae too numerous to mention, one of the most 
striking being the cosmopolitan Rhizocarpon geographicnm. 

Wainio 1 has described the lichens collected by Almquist at Pitlekai in 
N.E. Siberia just on the borders of the Arctic Circle, and he gives a vivid 
account of the general topography. The snow lies on the ground till June 
and falls again in September, but many lichens succeed in growing and 
fruiting. It is a region of tundra and sand, strewn more or less with stones. 
Most of the sand is bare of all vegetation; but where mosses, etc., have 
gained a footing, there are also a fair number of lichens : Lecanora tartarea, 
Psoroma hypnorum, with Lecideae, Parmeliae, Cladoniae, Stereocaulon alpinnm, 
Solortna crocea, SpJiaeropJwrus globosus, Alectoria nigricans and Gyrophora 
proboscidea. Some granite rocks in that neighbourhood rise to a height of 
200 ft., and though bare of vegetation on the north side, yet, in sheltered 
nooks, several species are to be found. Stunted bushes of willow grow 
here and there, and on these occur always the same species : Placodium 
ferrugineum, Rinodina archaea, Buellia myriocarpa and Arthopyrenia puncti- 
formis. Some species such as Sphaerophorus globosus, Dactylina arctica 
(a purely Arctic genus and species) and Thamnolia vermicularis are so. 
abundant that they bulk as largely as other better represented genera such 

1 Wainio 1909. 



DISTRIBUTION 347 

as Cladoniae, Lecanorae or Lecideae. On the soil, Lecanorae cover the largest 
areas. 

Wainio determined a large number of lichens with many new species, 
but the region is colder than that of Lappland, and trees with tree-lichens 
are absent, with the exception of those given above. In Arctic Siberia, 
Elenkin 1 discovered a new lichen Placodium subfruticulosum which scarcely 
differs from Darbishire's 2 Antarctic species PI. fruticulosnm (or /-*. regale); 
both are distinguished by the fruticose growth of the thallus, for which reason 
Hue 3 placed them in a new genus, Polycauliona. 

The Antarctic Zone and the neighbouring lands are less hospitable to 
plant life than the northern regions, and there is practically no accumulation 
of detritus. Collections have been made by explorers, and several lists have 
been published which include a marvellous number of species common to 
both Poles, if the subantarctic lands are included in the survey. An analytic 
study of the various lists has been published by Darbishire 4 . He recognizes 
1 06 true Antarctic lichens half of which are Arctic as well. The greater 
number are crustaceous and are plants common also to other lands though 
a certain number are endemic. The most abundant genera in species as 
well as individuals are Lecidea and Lecanora. Several bright yellow species 
of Placodium PI. elegans, PL murorum, etc., are there as at the North Pole. 
Among the larger forms, Parnieliae, Cetrariae, and Cladoniae are fairly 
numerous; Usneae and Rainalinae rather uncommon, while members of the 
Stictaceae are much more abundant than in the North. The common species 
of Peltigera also occur in Antarctica, though P. aphthosa and P. venosa are 
wanting ; both of these latter are boreal species. Darbishire adds that lichens 
have so great a capacity to withstand cold, that they are only checked by 
the snow covering, and were bare rocks to be found at the South Pole, he 
is sure lichens would take possession of them. The most southerly point 
at which any plant has been found is 78 South latitude and 162 East 
longitude, in which locality the lichen Lecanora subfusca was collected by- 
members of Scott's Antarctic expedition (1901-1904) at a height of 5000 ft. 

A somewhat different view of the Antarctic lichen flora is indicated by 
Hue 3 in his account of the plants brought back by the second French 
Antarctic Expedition. The collection was an extremely favourable and 
important one : great blocks of stone with their communities of lichens were 
secured, and these blocks were entirely covered, the crustaceous species, 
especially, spreading over every inch of space. 

Hue determined 126 species, but as 15 of these came from the Magellan 

regions only 1 1 1 were truly Antarctic. Of these 90 are new species, 29 of 

them belonging to the genus Buellia. Hue considers, therefore, that in 

Antarctica there is a flora that, with the exception of cosmopolitan species, 

1 Elenkin 1906. 2 Darbishire 1905. 3 Hue 1915. 4 Darbishire 1912. 



34 8 SYSTEMATIC 

is different from every other, and is special to these southern regions. Dar- 
bishire himself described 34 new Antarctic species, but only 10 of these 
are from true Antarctica; the others were collected in South Georgia, the 
Falkland Islands or Tierra del Fuego. Even though many species are 
endemic in the south, the fact remains that a remarkable number of lichens 
which occur intermediately on mountain summits are common to both Polar 
areas. 

C. LICHENS OF THE TEMPERATE ZONES 

Regions outside the Polar Circles which enjoy, on the whole, cool moist 
climates, are specially favourable to lichen growth, and the recorded numbers 
are very large. The European countries are naturally those in which the 
lichen flora is best known. Whereas polar and high Alpine species are 
stunted in growth and often sterile, those in milder localities grow and fruit 
well, and the more highly developed species are more frequent. Parmeliae, 
Nephromae, Usneae and Ramalinae become prominent, especially in the 
more northern districts. Many Arctic plants are represented on the higher 
altitudes. A comparison has been made between the lichens of Greenland 
and those of Germany: of 286 species recorded for the former country, 213 
have been found in Germany, the largest number of species common to 
both countries being crustaceous. Lindsay 1 considered that Greenland 
lichens were even more akin to those of Scandinavia. 

There is an astonishing similarity of lichens in the Temperate Zone all 
round the world. Commenting on a list of Chicago lichens by Calkins 2 , 
Hue 3 pointed out that with the exception of a few endemic species they 
resemble those of Normandy. The same result appears in Bruce Fink's 4 
careful compilation of Minnesota lichens, which may be accepted as typical 
of the Eastern and Middle States of North Temperate America. The 
genera from that region number nearly 70, and only two of these, Omphalaria 
and Heppia, are absent from our British Flora. The species naturally present 
much greater diversity. Very few Graphideae are reported. In other States 
of North America there occurs the singular aquatic lichen, Hydrothyria 
venosa, nearly akin to Peltigera. 

If we contrast American lichens with these collected in South Siberia 
near Lake Baikal 5 , we recognize there also the influence of temperate 
conditions. Several species of Usnea are listed, U. barbata, U. florida, 
U. hirta and U. longissima, all of them also American forms, U. longissima 
having been found in Wisconsin. Xanthoria parietina, an almost cosmo- 
politan lichen, is absent from this district, and is not recorded from Minnesota. 
The opinion 6 in America is that it is a maritime species: Tuckerman gives 

1 Lindsay 1870. 2 Calkins 1896. 3 Hue 1898. * Fink 1903. 

5 Wainio 1896. 6 Comm. Heber Howe. 



DISTRIBUTION 349 

its habitat as "the neighbourhood of a great water," and reports it from 
near Lake Superior. In our country it grows at a good distance from the 
sea, in Yorkshire dales, etc., but all our counties would rank as maritime 
in the American sense. Lecanora tartarea which is rare in Minnesota is also 
absent from the Lake Baikal region. It occurs frequently both in Arctic 
and in Antarctic regions, and is probably also somewhat maritime in habitat. 
Many of the Parmeliae, NepJiromiae and Peltigerae, common to all northern 
temperate climes, are Siberian as are also Cladoniae and many crustaceous 
species. There is only one Sticta, St. Wrightii, a Japanese lichen, recorded 
by Wainio from this Siberian locality. 

A marked difference as regards species is noted between the Flora of 
Minnesota and that of California. Herre 1 has directed attention to the 
great similarity between the lichens of the latter state and those of 
Europe : many European species occur along the coast and nowhere else 
in America so far as is yet known ; as examples he cites, among others, 
Calicium hyperellum, Lecidea quernea, L. aromatica, Gyrophora polyrhiza, 
Pertusaria amara, Roccella fuciformis, R. fucoides and R. tinctoria. The 
Scandinavian lichen, Letharia vulpina, grows abundantly there and fruits 
freely; it is very rare in other parts of America. Herre found, however, 
no specimens of Cladonia rangiferina, Cl. alpestris or CL syhatica, nor 
any species of Graphis\ he is unable to explain these anomalies in distri- 
bution, but he considers that the cool equable climate is largely responsible: 
it is so much more like that of the milder countries of Europe than of the 
states east of the Sierra Nevada. His contention is supported by a con- 
sideration of Japanese lichens. With a somewhat similar climate there is 
a great preponderance of European forms. Out of 382 species determined 
by Nylander 2 , 209 were European. There were 17 Graphideae, 31 Parmeliae, 
and 23 Cladoniae, all of the last named being European. These results of 
Nylander's accord well with a short list of 30 species from Japan compiled 
by Muller 3 at an earlier date. They were chiefly crustaceous tree-lichens; 
but the Cladoniae recorded are the familiar British species Cl. fimbriata, 
Cl. pyxidata and CL verticillata. 

With the Japanese Flora may be compared a list 4 of Maingay's lichens 
from China, 35 in all. Collema limosum, the only representative of Colle- 
maceae in the list, is European, as are the two species of Ramalina, R.graci- 
lenta and R. pollinaria ; four species of Physcia are European, the remaining 
Ph. picta being a common tropical or subtropical plant. Lecanora saxicola, 
L. cinerea, Placodium callopismnm and PI. citrinum are cosmopolitan, other 
Lecanorae and most of the Lecideae are new. Graphis scripta, Opegrapka 
subsiderella and Arthonia cinnabarina the few Graphideae collected are 

1 Herre 1910. 2 Nylander 1890. 3 Muller 1879. 

4 Nylander and Cromhie 1884. 



350 SYSTEMATIC 

more or less familiar home plants. Among the Pyrenocarpei, Verrucaria 
(Pyrenuld) nitida occurs ; it is a widely distributed tree-lichen. 

It is unnecessary to describe in detail the British lichens. Some districts 
have been thoroughly worked, others have barely been touched. The flora 
as a whole is of a western European type showing the influence of the Gulf 
Stream, though there is also a representative boreal growth on the moorlands 
and higher hills, especially in Scotland. Such species as Parmelia pubescens, 
P. stygia and P. alpicola recall the Arctic Circle while Alectoriae, Cetrariae 
and Gyrophorae represent affinity with the colder temperate zone. 

In the southern counties such species as Sticta aurata, S. damaecornis, 
Phaeographis Lyellii and Lecanora (Lecanid) holophaea belong to the flora 
of the Atlantic seaboard, while in S.W. Ireland the tropical genera Lepto- 
gidium and Anthracothedum are each represented by a single species. The 
tropical or subtropical genus Coenogonium occurs in Great Britain and in 
Germany, with one sterile species, C. ebeneum. Enterographa crassa is 
another of our common western lichens which however has travelled east- 
wards as far as Wiesbaden. Roccella is essentially a maritime genus of 
warm climates : two species, R.fuaformtsand R.fucoides, grow on our south 
and west coasts. The famous R. tinctoria is a Mediterranean plant, though 
it is recorded also from a number of localities outside that region and has 
been collected in Australia. 

In the temperate zones of the southern hemisphere are situated the great 
narrowing projections of South Africa and South America with Australia and 
New Zealand. As we have seen, the Antarctic flora prevails more or less in 
the extreme southern part of America, and the similarity between the lichens 
of that country and those of New Zealand is very striking, especially in the 
fruticulose forms. There is a very abundant flora in the New Zealand 
islands with their cool moist climate and high mountains. Churchill 
Babington 1 described the collections made by Hooker. Stirton 2 added 
many species, among others Calycidium cuneatum, evidently endemic. Later, 
Nylander 3 published the species already known, and Hellbom 4 followed 
with an account of New Zealand lichens based on Berggsen's collections ; 
many more must be still undiscovered. Especially noticeable as compared 
with the north, are the numbers of Stictaceae which reach their highest 
development of species and individuals in Australasia. They are as numerous 
and as prominent as are Gyrophoraceae in the north. A genus of Parmelia- 
ceae, Hetorodea, which, like the Stictae, bears cyphellae on the lower surface, 
is peculiar to Australia. 

A warm current from the tropical Pacific Ocean passes southwards along 
the East Coast of Australia, and Wilson 6 has traced its influence on the 

1 Babington 1855. 2 Stirton 1875. 3 Nylander 1888. 4 Hellbom 1896. 

5 Wilson 1892. 



DISTRIBUTION 351 

lichens of Australia and Tasmania to which countries a few tropical species 
of Graphis, Chiodecton and Trypethelium have migrated. Various unusual 
types are to be found there also: the beautiful Cladonia retepora (Fig. 71), 
which spreads over the ground in cushion-like growths, with the genera 
Thysanothecium and Neophyllis, genera of Cladoniaceae endemic in these 
regions. 

The continent of Africa on the north and east is in so close connection 
with Europe and Asia that little peculiarity in the flora could be expected. 
In comparing small representative collections of lichens, 37 species from 
Egypt and 20 from Palestine, Miiller 1 found that there was a great affinity 
between these two countries. Of the Palestine species, eight were cosmo- 
politan ; among the crustaceous genera, Lecanorae were the most numerous. 
There was no record of new genera. 

The vast African continent more especially the central region has 
been but little explored in a lichenological sense; but in 1895 Stizenberger 2 
listed all of the species known, amounting to 1 593, and new plants and new 
records have been added since that day. The familiar genera are well 
represented, Nephromium, Xanthoria, Physcia, Parmelia, Ranialina and 
Roccella, some of them by large and handsome species. In the Sahara 
Steiner' found that genera with blue-green algae such as the Gloeolichens 
were particularly abundant ; Heppia and Endocarpon were also frequent. 
Algeria has a Mediterranean Flora rather than tropical or subtropical. 
Flagey 4 records no species of Graphis for the province of Constantine, and 
only 22 species of other Graphideae. Most of the 519 lichens listed by him 
there are crustaceous species. South America stretches from the Tropics 
in the north to Antarctica in the south. Tropical conditions prevail over 
the central countries and tropical tree-lichens, Graphidaceae.Thelotremaceae, 
etc. are frequent ; further West, on the Pacific slopes, Usneae and Ramalinae 
hang in great festoons from the branches, while the foliose Parmeliae and 
Stictae grow to a large size on the trunks of the trees. 

Wainio's 8 Lie/tens du Bresil is one of the classic systematic books and 
embodies the writer's views on lichen classification. There are no new 
families recorded though a number of genera and many species are new, 
and, so far as is yet known, these are endemic. Many of our common forms 
are absent ; thus Peltigera is represented by three species only, P. leptoderma, 
P. spuriella and P. Americana, the two latter being new species. Sticta 
(including Stictina) includes only five species, and Coenogonium three. There 
are 39 species of P armelia with 33 of Lecanora and 68 of Lecidea, many of 
them new species. 

1 Miiller- Argau 1884. * Stizenberger 1888-1895. 3 Steiner 1895. 

4 Flagey 1892. 5 Wainio 1890. 



352 SYSTEMATIC 

D. LICHENS OF TROPICAL REGIONS 

In the tropics lichens come under the influence of many climates : on 
the high mountains there is a region of perpetual snow, lower down a gradual 
change to temperate and finally to tropical conditions of extreme heat, and, 
in some instances, extreme moisture. There is thus a bewildering variety 
of forms. By "tropical" however the warmer climate is always implied. 
Several families and genera seem to flourish best in these warm moist 
conditions and our familiar species grow there to a large size. Among 
crustaceous families Thelotremaceae and Graphidaceae are especially abun- 
dant, and probably originated there.. In the old comprehensive genus 
Graphis, 300 species were recorded from the tropics. It should be borne in 
mind that Trentepohlia, the alga that forms the gonidia of these lichens, is 
very abundant in the tropics. Coenogonium, a genus containing about twelve 
species and also associated with Trentepohlia, is scarcely found in Europe, 
except one sterile species, C. ebenenm. Other species of the genus have been 
recorded as far north as Algeria in the Eastern Hemisphere and Louisiana 
in the Western, while one species, C. implexum, occurs in the southern 
temperate zone in Australia and New Zealand. 

Of exclusively tropical lichens, the Hymenolichens are the most note- 
worthy. They include three genera, Cora, Corella and Dictyonema, the few 
species of which grow on trees or on the ground both in eastern and western 
tropical countries. 

Other tropical or subtropical forms are Oropogon loxensis, similar to 
Alectoria in form and habit, but with one brown muriform spore in the 
ascus; it is only found in tropical or subtropical lands. Physcidia Wrightii 
(Parmeliaceae) is exclusively a Cuban lichen. Several small genera of 
Pyrenopsidaceae such asjenmania (British Guiana), Paulia (Polynesia) and 
Phloeopeccania (South Arabia) seem to be confined to very hot localities. 
On the other hand Collemaceae are rare : Wainio records from Brazil only 
four species of Collema, with nine of Leptogium. 

Among Pyrenolichens, Paratheliaceae, Mycoporaceae and Astrothe- 
liaceae are almost exclusively of tropical distribution, and finally the leaf 
lichens with very few exceptions. These follow the leaf algae, Mycoidea, 
Phycopeltis, etc., which are so abundant on the coriaceous long-lived green 
leaves of a number of tropical Phanerogams. All the Strigulaceae are 
epiphytic lichens. Phyllophthalmaria (Thelotremaceae) is also a leaf genus; 
one of the species, Ph. coccinea, has beautiful carmine-red apothecia. The 
genera of the tropical family Ectolechiaceae also inhabit leaves, but they 
are associated with Protococcaceae ; one of the genera Sporopodium 1 is re- 
markable as having hymenial gonidia. Though tropical in the main, 

1 Wainio 1890, II. p. 27 (recorded under Lecided). 



DISTRIBUTION 353 

epiphyllous lichens may spread to the regions beyond: Sforopodium 
Caucasium and a sterile Strigula were found by Elenkin and Woronichin 1 
on leaves of Buxus sempervirens in the Caucasus, well outside the tropics. 

Pilocarpon, an epiphytic genus, is associated with Protococcaceae ; one 
of the species, P. leucoblepharum, spreads from the bark to the leaves of pine- 
trees ; it is widely distributed and has also been reported in the Caucasus". 
Ckrysothrix, in which the gonidia belong to the algal genus Pa/mella, grows on 
Cactus spines in Chili, and may also rank as a subtropical epiphyllous lichen. 

A series of lichens from the warm temperate region of Transcaucasia 
investigated by Steiner 3 were found to be very similar to those of Central 
Europe. Lecanoraceae were, however, more abundant than Lecideaceae 
and Verrucariaceae were comparatively rare. 

Much of Asia lies within tropical or subtropical influences. Several 
regions have received some amount of attention from collectors. From 
Persia there has been published a list of 59 species determined by Miiller 4 ; 
several of them are Egyptian or Arabian plants, 1 5 are new species, but the 
greater number are European. 

A small collection of 53 species from India, near to Calcutta, published 
by Nylander 5 , included a new genus of Caliciaceae, Pyrgidium (P. bengalense), 
allied to Sphinctrina. He also recorded Ramalina angulosa in African species, 
along with R. calicaris, R. farinacea and Parmelia perlata, f. isidiophora, 
which are British. Other foliose forms, Physcia picta, Pyxine Cocoes and 
P. Meissnerii are tropical or subtropical ; along with these were collected 
crustaceous tropical species belonging to Lecanorae, Lecideae, Graphideae, etc. 

Leighton 6 published a collection of Ceylon lichens and found that Gra- 
phideae predominated. Nylander 7 came to the same conclusion with regard 
to lichens referred to him: out of 159 species investigated from Ceylon, 
there were 36 species of Graphideae. In another list 8 of Labuan, Singapore 
and Malacca lichens, 164 in all, he found that 56 belonged to the Graphidei, 
36 to Pyrenocarpei, 14 to Thelotremei and n to Parmelei; only 15 species 
were European. 

On the whole it is safe to conclude from the above and other publications 
that the exceptional conditions of the tropics have produced many distinc- 
tive lichens, but that a greater abundance both of species and individuals is 
now to be found in temperate and cold climates. 

III. FOSSIL LICHENS 

In pronouncing on the great antiquity of lichens, proof has been adduced 
from physiological rather than from phytogeological evidence. It would 
have been of surpassing interest to trace back these plants through the ages, 

1 Elenkin and Woronichin 1908. 2 Jaczewski 1904. 3 Steiner 1919. 4 MUller 1891. 

5 Nylander 1867. 6 Leighton 1869. ' Nylander 1900. 8 Nylander 1891. 

s. L. 2 3 



354 SYSTEMATIC 

even if it were never possible to assign to any definite period the first 
symbiosis of the fungus and alga ; but among fossil plants there are only 
scanty records of lichens and even these few are of doubtful determination. 

The reason for this is fairly obvious : not only are the primitive thalline 
forms too indistinct for recognizable preservation, but all lichens are charac- 
terized by the gelatinous nature of the hyphal or of the algal membranes 
which readily imbibe water. They thus become soft and flaccid and unfit 
to leave any impress on sedimentary rocks. It has also been pointed out by 
Schimper 1 that while deciduous leaves with fungi on them are abundant in 
fossil beds, lichens are entirely wanting. These latter are so firmly attached 
to the rock's or trees on which they grow that they are rarely dislodged, and 
form no part of wind- or autumn-fall. Trunks and branches of trees lose 
their bark by decay long before they become fossilized and thus all trace of 
their lichen covering disappears. 

The few records that have been made are here tabulated in chronological 
order: 

1. PALAEOZOIC. Schimper decides that there are no records of lichens 
in the earlier epochs. Any allusions 2 to their occurrence are held to be ex- 
tremely vague and speculative. 

2. MESOZOIC. Braun 3 has recorded a Ramalinites lacerns from the 
Keuper sandstone at Eckersdorf, though later 4 he seemed to be doubtful as 
to his determination. One other lichen, an Opegrapha, has been described 5 
from the chalk at Aix. 

3. CAINOZOIC. In the brown-coal formations of Saxony Engelhardt 6 
finds two lichens : Ramalina tertiaria, a much branched plant, the fronds 
being flat and not channelled " and of further interest that it is attached to 
a carbonized stem." The second form, Lichen dichotomies, has a dichoto- 
mously branching strap-shaped frond. " There is sufficient evidence that 
these fronds were cylindrical and that the width is due to pressure. In one 
place a channel is visible, filled with an ochraceous yellow substance." 

Other records on brown coal or lignite are : Verrncarites geanthricis" 1 
Goepp., somewhat similar to Pyrenida nitida, found at Muskau in Silesia ; 
Opegrapha Thomasiana* Goepp., near to Opegrapha varm,a.nd Graphis scripta 
succinea Goepp. 9 on a piece of lignite in amber beds, all of them doubtful. 

Schimper has questioned, as he well might, Ludwig's 10 records from 
lignite from the Rhein-VVetterau Tertiary formations ; these are : Cla- 
donia rosea, Lichen albineus, L. diffissus and L. orbiculatus ; he thinks they 
are probably fungus mycelia. Another lichen, a Parmelia with apothecia, 

1 Schimper 1869, p. 145. 2 Lindsay 1879. :i Braun 1840. * Muenster 1846, p. 26. 

5 Eltingshausen and Debey 1857. 6 Engelhardt 1870 (PI. I. figs, i and 2). 

7 Goeppert 1845, p. 195. 8 See Schimper 1869, pp. 145, etc. 

"Goeppert and Menge 1883, t. i, fig. 3. 10 Ludwig 1859, p. 61 (t. 9, figs. 1-4), 1859-61. 



FOSSIL LICHENS 355 

which recalls somewhat P. saxatilis or P. conspersa, collected by Geyler 
also in the brown coal of Wetterau is accepted by Schimper 1 as more trust- 
worthy. 

More authentic also are the lichens from the amber beds of Konigsberg 
and elsewhere collected by Goeppert and others. These deposits are 
Cainozoic and have been described by Goeppert and Menge 2 as middle 
Miocene. Schimper gives the list as: Parmelia lacnnosa Meng. and Goepp., 
fragments of thallus near to P. saxatilis; Sphaerophornscoralloides; Cladonia 
divaricata Meng. and Goepp.; Cl. furcata; Ramalina calicaris \zrs.fraxinea 
and canaliculata ; Cornicularia aculeata, C. subpubescens Goepp., C, ochroleiica, 
C. succinea Goepp., and Usnea barbata var. hirta. Schimper rather deprecates 
specific determinations when dealing with such imperfect fragments. 

In a later work Goeppert and Menge 2 state that they have found twelve 
different amber lichens and that among these are Physcia ciliaris, Parmelia 
physodes and Graphis (probably G. scripta succinea) along with Peziza retinae 
which is more generally classified among lichens as Lecidea (Biatorelld} 
resinae. 

Another series of lichens found in recent deposits in North Europe has 
been described by Sernander 3 as "subfossil." While engaged on the investi- 
gations undertaken by the Swedish Turf-Moor Commission, he noted the 
alternation of slightly raised Sphagnum beds with lower-lying stretches of 
Calluna and lichen moor in some instances dense communities of Cladonia 
rangiferina. In time the turf-forming Sphagnum overtopped and invaded 
the drier moorland, covering it with a new formation of turf. Beneath these 
layers of " regenerated turf" were found local accumulations of blackened 
remains of the Cladonia still recognizable by the form and branching. Some 
specimens of Cetraria islandica were also determined. 

Of especial lichenological interest in these northern regions was the 
Calcareous Tufa or Calc-sinter in which Sernander also found subfossil 
lichens distinct impressions of Peltigera spp. and the foveolae of endolithic 
calcicolous species. 

In another category he has placed Ramalina fraxinea, Graphis sp. and 
Opegrapha sp., traces of which were embedded with drift in the Tufa. In 
the two Graphideae the walls of apothecia and pycnidia were preserved. 
Sernander considers their presence of interest as testifying to warmer con- 
ditions than now prevail in these latitudes. 

' Schimper in Zittel 1890. 2 Goeppert and Menge 1883. 3 Sernander 1918. 



232 



CHAPTER IX 

ECOLOGY 
A. GENERAL INTRODUCTION 

ECOLOGY is the science that deals with the habitats of plants and their 
response to the environment of climate or of substratum. Ecology in the 
lichen kingdom is habitat "writ large," and though it will not be possible in 
so wide a field to enter into much detail, even a short examination of lichens 
in this aspect should yield interesting results, especially as lichens have 
never, at any time, been described without reference to their habitat. In 
very early days, medicinal Usneas were supposed to possess peculiar virtues 
according to the trees on which they grew and which are therefore carefully 
recorded, and all down the pages of lichen literature, no diagnosis has been 
drawn up without definite reference to the nature of the substratum. Not 
only rocks and trees are recorded, but the kind of rock and the kind of tree 
are often specified. The important part played by rock lichens in preparing 
soil for other plants has also received much attention 1 . 

Several comprehensive works on Ecology have been published in recent 
times and though they deal mainly with the higher vegetation, the general 
plan of study of land plants is well adapted to lichens. A series of definitions 
and explanations of the terms used will be of service : 

Thus in a work by Moss 2 we read " The flora is composed of the indi- 
vidual species: the vegetation comprises the groupings of these species into 
ensembles termed vegetation units or plant communities." And again : 

1. "A plant formation is the whole of the vegetation which occurs on 
a definite and essentially uniform habitat." All kinds of plants are included 
in the formation, so that strictly speaking a lichen formation is one in which 
lichens are the dominant plants. Cf. p. 394. The term however is very loosely 
used in the literature. A uniform habitat, as regards lichens, would be that 
of the different kinds of soil, of rock, of tree, etc. 

2. "A. plant association is of lower rank than a formation, and is charac- 
terized by minor differences within the generally uniform habitat." It 
represents a more limited community within the formation. 

3. " A plant society is of lower rank than an association, and is marked 
by still less fundamental differences of the habitat." The last-named term 
represents chiefly aggregations of single species. Moss adds that: ''plant 
community is a convenient and general term used for a vegetation unit of 
any rank." 

Climatic conditions and geographical position are included in any con- 
sideration of habitat, as lichens like other plants are susceptible to external 
influences. 

1 See p. 392. 2 Moss 1913. 



GENERAL INTRODUCTION 357 

Ecological plant-geography has been well defined by Macmillan 1 as 
"the science which treats of the reciprocal relation between physiographic 
conditions and life requirements of organisms in so far as such relations 
manifest themselves in choice of habitats and method of establishment 
upon them... resulting in the origin and development of plant formations." 

B. EXTERNAL INFLUENCES 

The climatic factors most favourable to lichen development are direct 
light (already discussed) 2 , a moderate or cold temperature, constant moisture 
and a clear pure atmosphere. Wind also affects their growth. 

a. TEMPERATURE. Lichens, as we have seen, can endure the heat of 
direct sunlight owing to the protection afforded by thickened cortices, colour 
pigments, etc. Where such heat is so intense as to be injurious the gonidia 
succumb first :i . 

Lichens endure low temperatures better than other plants, their xerophytic 
structure rendering them proof against extreme conditions: the hyphae 
have thick walls with reduced cell lumen and extremely meagre contents. 
Freezing for prolonged periods does them little injury ; they revive again 
when conditions become more favourable. Efficient protection is also afforded 
by the thickened cortex of such lichens as exist in Polar areas, or at high 
altitudes. Thus various species of Cetrariae with a stout "decomposed" 
amorphous cortex can withstand very low temperatures and grow freely on the 
tundra, while Cladonia rangiferina, also a northern lichen, but without a con- 
tinuous cortex, cannot exist in such cold conditions, unless in localities where 
it is protected by a covering of snow during the most inclement seasons. 

b. HUMIDITY. A high degree of humidity is distinctly of advantage to 
the growth of the lichen thallus, though when the moist conditions are ex- 
cessive the plants become turgid and soredial states are developed. 

The great abundance of lichens in the western districts of the British 
Isles, where the rainfall is heaviest, is proof enough of the advantage of 
moisture, and on trees it is the side exposed to wind and rain that is most 
plentifully covered. A series of observations on lichens and rainfall were 
made by West 4 and have been published since his death. He has remarked 
in more than one of his papers that a most favourable situation for lichen 
growth is one that is subject to a drive of wind with much rain. In localities 
with an average of 216 days of rain in the year, he found abundant and 
luxuriant growths of the larger foliose species. In West Ireland there were 
specimens oiRicasolia laetevirens measuring 1 65 by 60 cm. I n West Scotland 
with an "average of total days of rain, 225," he found plants of Ricasolia am- 
plissima 150 x 90 cm. in size, of R. laetevirens 120x90 cm., while Pertusaria 

1 Macmillan 1894. - See p. 240 et seq. 3 See p. 238. 4 West 1915- 



358 ECOLOGY 

globulifera formed a continuous crust on the trees as much as 120 x 90 cm. 
Lecanora tartarea seemed to thrive exceptionally well when subject to 
driving mists and rains from mountain or moorland, and was in these cir- 
cumstances frequently the dominant epiphyte. Bruce Fink 1 also observed 
in his ecological excursions that the number of species and individuals was 
greater near lakes or rivers. 

Though a fair number of lichens are adapted to life wholly or partly 
under water, land forms are mostly xerophytic in structure, and die off if 
submerged for any length of time. The Peltigerae are perhaps the most 
hydrophilous of purely land species. Many Alpine or Polar forms are 
covered with snow for long periods. In the extreme north it affords more 
or less protection; and Kihlman 2 and others have remarked on the scarcity 
of lichens in localities denuded of the snow mantle and exposed to severe 
winter cold. On the other hand lichens on the high Alpine summits that are 
covered with snow the greater part of the year suffer, according to Nilson 3 , 
from the excessive moisture and the deprivation of light. Foliose and 
fruticose forms were, he found, dwarfed in size; the crustaceous species had 
a very thin thallus and in all of them the colour was impure. Gyrophorae 
seemed to be most affected : folds and outgrowths of the thallus were formed 
and the internal tissues were partly disintegrated. Lichens on the blocks 
of the glacier moraines which are subject to inundations of ice-cold water 
after the snow has melted, were unhealthy looking, poorly developed and 
often sterile, though able to persist in a barren state. Lindsay 4 noted as 
a result of such conditions on Cladoniae not only sterility but also de-. 
formity both of vegetative and reproductive organs ; discolouration and 
mottling of the thallus and an increased development of squamules of the 
primary thallus and on the podetia. 

c. WIND. Horizontal crustaceous or foliose lichens are not liable to 
direct injury by wind as their close adherence to the substratum sufficiently 
shelters them. It is only when the wind carries with it any considerable 
quantity of sand that the tree or rock surfaces are swept bare and prevented 
from ever harbouring any vegetation, and also, as has been already noted, 
the terrible winds round the poles are fatal to lichens exposed to the 
blasts unless they are provided with a special protective cortex. After 
crustaceous forms, species of Cetraria, Stereocaulon and Cladonia are best 
fitted for weathering wind storms: the tufted 5 cushion-like growth adopted 
by these lichens gives them mutual protection, not only against wind, but 
against superincumbent masses of snow. Kihlman 2 has given us a vivid 
account of wind action in the Tundra region. He noted numerous hollows 
completely scooped out down to the sand : in these sheltered nooks he 

1 Fink 1894. 2 Kihlman 1890. 3 Nilson 1907. 

4 Lindsay 1869. 6 Sattler 1914. 



EXTERNAL INFLUENCES 359 

observed the gradual colonization of the depressions, first by a growth of 
hepatics and mosses and by such ground lichens as Peltigera canina, P. 
aphthosa and Nephromium arcticum ; they cover the soil and in time the 
hollow becomes filled with a mass of vegetation consisting of Cladonias, 
mosses, etc. On reaching a certain more exposed level these begin to wither 
and die off at the tips, killed by the high cold winds. Then arrives Lecanora 
tartarea, one of the commonest Arctic lichens, and one which is readily 
a saprophyte on decayed vegetation. It covers completely the mound of 
weakened plants which are thus smothered and finally killed. The collapse 
of the substratum entails in turn the breaking of the Lecanora crust, and 
the next high wind sweeps away the whole crumbling mass. How long 
recolonization takes, it was impossible to find out. 

Upright fruticose lichens are necessarily more liable to damage by wind, 
but maritime Ramalinae and Roccellae do not seem to suffer in temperate 
climates, though in regions of extreme cold fruticose forms are dwarfed and 
stunted. The highest development of filamentous lichens is to be found in 
more or less sheltered woods, but the effect of wind on these lichens is not 
wholly unfavourable. Observations have been made by Peirce 1 on two 
American pendulous lichens which are dependent on wind for dissemina- 
tion. On the Californian coasts a very large and very frequent species, 
Ramalina reticulata (Fig. 64), is seldom found undamaged by wind. In 
Northern California the deciduous oaks Quercus alba and Q. Douglasii are 
festooned with the lichen, while the evergreen " live oak," Q. chrysolepis, 
with persistent foliage, only bears scraps that have been blown on to it. 
Nearer the coast and southward the lichen grows on all kinds of trees and 
shrubs. The fronds of this Ramalina form a delicate reticulation and when 
moist are easily torn. In the winter season, when the leaves are off the 
trees, wind- and rain-storms are frequent ; the lichen is then exposed to 
the full force of the elements and fragments and shreds are blown to other 
trees, becoming coiled and entangled round the naked branches and barky 
excrescences, on which they continue to grow and fruit perfectly well. 
A succeeding storm may loosen them and carry them still further. Peirce 
noted that only plants developed from the spore formed hold-fasts and 
they were always small, the largest formed measuring seven inches in length. 
Both the hold-fast and the primary stalk were too slight to resist the tearing 
action of the wind. 

Schrenk 2 made a series of observations and experiments with the lichens 
Usneaplicata and U. dasypoga, long hanging forms common on short-leaved 
conifers such as spruce and juniper. The branches of these trees are often 
covered with tangled masses of the lichens not due to local growth, but to 
wind-borne strands and to coiling and intertwining of the filaments owing 

1 Peirce 1898. 2 Schrenk 1898. 



360 ECOLOGY 

to successive wetting and drying. Tests were made as to the force of wind 
required to tear the lichens and it was found that velocities of 77 miles per 
hour were not sufficient to cause any pieces of the lichen to fly off when it 
was dry; but after soaking in water, the first pieces were torn off at 50 miles 
an hour. These figures are, however, considered by Schrenk to be too high 
as it was found impossible in artificially created wind to keep up the condi- 
tion of saturation. It is the combination of wind and rain that is so effective 
in ensuring the dispersal of both these lichens. 

d. HUMAX AGENCY. Though lichens are generally associated with un- 
disturbed areas and undisturbed conditions, yet accidents or convulsions of 
nature, as well as changes effected by man, may at times prove favourable 
to their development. The opening up of forests by thinning or clearing 
will be followed in time by a growth of tree and ground forms; newly 
planted trees may furnish a new lichen flora, and the building of houses 
and walls with their intermixture of calcareous mortar will attract a par- 
ticular series of siliceous or of lime-loving lichens. A few lichens are partial 
to the trees of cultivated areas, such as park-lands, avenues or road-sides. 
Among these are several species of Physcia : Ph. pulverulenta, Ph. ciliaris 
and Ph. stellaris, some species of Placodinm, and those lichens such as 
Lecanora varia that frequently grow on old palings. 

On the other hand lichens are driven away from areas of dense popula- 
tion, or from regions affected by the contaminated air of industrial centres. 
In our older British Floras there are records of lichens collected in London 
during the eighteenth century in Hyde Park and on Hampstead Heath but 
these have long disappeared. A variety of Lecanora galactina seems to be 
the only lichen left within the London district : it has been found at Camden 
Town, Netting Hill and South Kensington. 

So recently as 1866, Nylander 1 made a list of the lichens growing in the 
Luxembourg gardens in Paris; the chestnuts in the alley of the Observatory 
were the most thickly covered, and the list includes about 35 different 
species or varieties, some of them poorly developed and occurring but rarely, 
others always sterile, but quite a number in healthy fruiting condition. All 
of them were crustaceous or squamulose forms except Parmelia acetabulum, 
which was very rare and sterile; Physcia obscura var. and Ph. pulverulenta 
var., also sterile; Physcia stellaris with occasional abortive apothecia and 
Xantlwria parietina, abundant and fertile. In 1898, Hue 2 tells us, there 
were no lichens to be found on the trees and only traces of lichen growth 
on the stone balustrades. 

The question of atmospheric pollution in manufacturing districts and its 
effect on vegetation, more especially on lichen vegetation, has received 
special attention from Wheldon and Wilson 1 in their account of the lichens of 
1 Nylander 1866. * Hue 1898. * Wheldon and Wilson 1915. 



EXTERNAL INFLUENCES 361 

South Lancashire, a district peculiarly suitable for such an inquiry,as nowhere, 
according to the observations, are the evil effects of impure air so evident 
or so wide-spread. The unfavourable conditions have prevailed for a long 
time and the lichens have consequently become very rare, those that still 
survive leading but a meagre existence. The chief impurity is coal smoke 
which is produced not only from factories but from private dwellings, and 
its harmful effect goes far beyond the limits of the towns or suburbs, lichens 
being seen to deteriorate as soon as there is the slightest deposition of coal 
combustion products especially sulphur compounds either on the plants 
or on the surfaces on which they grow. The larger foliose and fruticose 
forms have evidently been the most severely affected. "While genera of 
bark-loving lichens such as Calicimn, Usnea, Ramalina, Grapliis, Opegrapha, 
Arthonia etc. are either wholly absent or are poorly represented in the 
district," corticolous species now represent about 15 per cent, of those that 
are left; those that seem best to resist the pernicious influences of the smoky 
atmosphere are, principally, Lecanora varia, Parmelia saxatilis,P.pJiysodes and 
to a less degree P. sulcata, P.fuliginusa var. laetevirens and Pcrtusaria ainara. 

Saxicolous lichens have also suffered severely in South Lancashire; not 
only the number of species, but the number of individuals is enormously 
reduced and the specimens that have persisted are usually poorly developed. 
The smoke-producing towns are situated in thevalley-bottoms.andthe smoke 
rises and drifts on to the surrounding hills and moorlands. The authors 
noted that crustaceous rock-lichens were in better condition on horizontal 
surfaces such as the copings of walls, or half-buried stones, etc. than on the 
perpendicular or sloping faces of rocks or walls. This was probably due 
to what they observed as to the effect of water trickling down the inclined 
substrata and becoming charged with acid from the rock surfaces. They 
also observed further that a calcareous substratum seemed to counteract the 
effect of the smoke, the sulphuric acid combining with the lime to form 
calcium sulphate, and the surface-washings thus being neutralized, the 
lichens there are more favourably situated. They found in good fruiting 
condition, on mortar, cement or concrete, the species Lecanora urbana, 
L. campestris, L. crenulata, Verrncaria mitralis, V. rupestris, Thelidinin 
microcarpum and StaurotJiele hymenogonia. Some of these occurred on the 
mortar of sandstone walls close to the town, "whilst on the surface of the 
sandstone itself no lichens were present." 

Soil-lichens were also strongly affected, the Cladoniae of the moorlands 
being in a very depauperate condition, and there was no trace of Stereocanlon 
or of Sphatropliorns species, which, according to older records, previously 
occurred on the high uplands. 

The influence of human agency is well exemplified in one of the London 
districts In 1883 Crombie published a list of the lichens recorded from 



362 ECOLOGY 

Epping Forest during the nineteenth century. They numbered 171 species, 
varieties or forms, but, at the date of publication, many had died out owing 
to the destruction of the older trees ; the undue crowding of the trees that 
were left and the ever increasing population on the outskirts of the Forest. 
Crombie himself made a systematic search for those that remained, and 
could only find some 85 different kinds, many of them in a fragmentary or 
sterile condition. 

R. Paulson and P. Thompson 1 commenced a lichen exploration of the 
Forest 27 years after Crombie's report was published, and they have found 
that though the houses and the population have continued to increase round 
the area, the lichens have not suffered. " Species considered by Crombie as 
rare or sterile are now fairly abundant, and produce numerous apothecia. 
Such are Baeomyces rufus, B. roseus, Cladonia pyxidata, Cl. macilenta var. 
coronata, Cl. Floerkeana f. trachypoda, Lecanora varia, Lecidea decolorant and 
Lecidea tricolor? They conclude that "some at least of the Forest lichens 
are in a far more healthy and fertile condition than they were 27 years ago." 
They attribute the improvement mainly to the thinning of trees and the 
opening up of glades through the Forest, letting in light and air not only to 
the tree trunks but to the soil. In 191 2 2 the authors in a second paper 
reported that 109 different kinds had been determined, and these, though 
still falling far short of the older lichen flora, considerably exceed the list 
of 85 recorded in 1883. 

C. LICHEN COMMUNITIES 

Lichen communities fall into a few definite groups, though, as we shall 
see, not a few species may be found to occur in several groups species 
that have been designated by some workers as "wanderers." The leading 
communities are : 

1. ARBOREAL, including those that grow on leaves, bark or wood. 

2. TERRICOLOUS, ground-lichens. 

3. SAXICOLOUS, rock-lichens. 

4. OMNICOLOUS, lichens that can exist on the most varied substrata, such 
as bones, leather, iron, etc. 

5. LOCALIZED COMMUNITIES in which owing to special conditions the 
lichens may become permanent and dominant. 

In all the groups lichens are more or less abundant. In arboreal and 
terricolous formations they may be associated with other plants; in saxi- 
colous and omnicolous formations they are the dominant vegetation. It will 
be desirable to select only a few of the typical communities that have been 
observed and recorded by workers in various lands. 

1 Paulson and Thompson 191 1. * Paulson and Thompson 1917. 



LICHEN COMMUNITIES 363 

I. ARBOREAL 

Arboreal communities may be held to comprise those lichens that grow 
on wood, bark or leaves. They are usually the dominant and often the sole 
vegetation, but in some localities there may be a considerable development 
of mosses, etc., or a mantle of protococcaceous algae may cover the bark. 
Certain lichens that are normally corticolous may also be found on dead 
wood or may be erratic on neighbouring rocks : Usnea florida for instance 
is a true corticolous species, but it grows occasionally on rocks or boulders 
generally in crowded association with other foliose or fruticose lichens. 

Most of the larger lichens are arboreal, though there are many excep- 
tions : Parmelia perlata develops to a large size on boulders as well as on 
trees ; some species of Ramalinae are constantly saxicolous while there are 
only rare instances of Roccellae that grow on trees. The purely tropical or 
subtropical genera are corticolous rather than saxicolous, but species that 
have appeared in colder regions may have acquired the saxicolous habit : 
thus Coenogonium in the tropics grows on trees, but the European species, 
C. ebeneum, grows on stone. 

a. EPIPHYLLOUS. These grow on Ferns or on the coriaceous leaves of 
evergreens in the tropics. Many of them are associated with Phycopeltis, 
Phyllactidium or Mycoidea, and follow in the wake of these algae. Obser- 
vations are lacking as to the associations or societies of these lichens whether 
they grow singly or in companies. The best known are the Strigulaceae : 
there are six genera in that family, and some of the species have a wide 
distribution. The most frequent genus is Strigula associated with Phyco- 
peltis which forms round grey spots on leaves, and is almost entirely confined 
to tropical regions. Chodat 1 records a sterile species, 5. Buxi, on box leaves 
from the neighbourhood of Geneva. 

Other genera, such as those of Ectolechiaceae, which inhabit fern scales 
and evergreen leaves, are associated with Protococcaceae. Pilocarpon leuco- 
blepharum with similar gonidia grows round the base of pine-needles. It is 
found in the Caucasus. In our own woods, along the outer edges, the lower 
spreading branches of the fir-trees are often decked with numerous plants 
of Parmelia physodes, a true " plant society," but that lichen is a confirmed 
"wanderer." Biatorina Bouteillei, on box leaves, is a British and Continental 
lichen. 

b. CORTICOLOUS. In this series are to be found many varying groups, 
the type of lichen depending more on the physical nature of the bark than 
on the kind of trees. Those with a smooth bark such as hazel, beech, lime, 
etc., and younger trees in general, bear only crustaceous species, many of 
them with a very thin thallus, often partly immersed below the surface. 

1 Chodat 1912. 



364 ECOLOGY 

As the trees become older and the bark takes on a more rugged character, 
other types of lichens gain a foothold, such as the thicker crustaceous forms 
like Pertusaria, or the larger foliose and fruticose species. The moisture that 
is collected and retained by the rough bark is probably the important factor 
in the establishment of the thicker crusts, and, as regards the larger lichens, 
both rhizinae and hold-fasts are able to gain a secure grip of the broken-up 
unequal surface, such as would be quite impossible on trees with smooth bark. 

Among the first t6 pay attention to the ecological grouping of corticolous 
lichens was A. L. Fee 1 , a Professor of Natural Science and an Army doctor, 
who wrote on many literary and botanical subjects. In his account of the 
Cryptogams that grow on "officinal bark," he states that the most lichenized 
of all the Cimhotiae was the one known as " Loxa," the bark of which was 
covered with species of Parmelta, Sticta and Usnea along with crustaceous 
forms of Lecanora, Lecidea, Graphis and Verrucaria. Another species, Cin- 
chona cordifolia, was completely covered, but with crustaceous forms only : 
species of Graphidaceae, Lecanora and Lecidea were abundant, but Trype- 
thelium, Chiodecton, Pyrenula and Verrucaria were also represented. On each 
species of tree some particular lichen was generally dominant: 
A species of Thelotrema on Cinchona oblongifolia. 
A species of Chiodecton on C. cordifolia. 
A species of Sarcographa on C. condaminea. 

Fries 2 , in his geography of lichens, distinguished as arboreal and "hypo- 
phloeodal" species of Verrucariaceae, while the Graphideae, which also grew 
on bark, were erumpent. Usnea barbata, Evernia prunastri, etc., though grow- 
ing normally on trees might, he says, be associated with rock species. 

More extensive studies of habitat were made by Krempelhuber 3 in his 
Bavarian Lichens. In summing up the various "formations" of lichens, he 
gives lists of those that grow, in that district, exclusively on either coniferous 
or deciduous trees, with added lists of those that grow on either type of tree 
indifferently. Among those found always on conifers or on coniferous wood 
are : Letharia vulpina, Cetraria Laureri, Pannelia aleurites and a number of 
crustaceous species. Those that are restricted to the trunks and branches of 
leafy trees are crustaceous with the exception of some foliose Collemaceae 
such as Leptogium Hildenbrandii, Collema nigrescens, etc. 

Arnold 4 carried to its furthest limit the method of arranging lichens 
ecologically, in his account of those plants from the neighbourhood of 
Munich. He gives " formation " lists, not only for particular substrata and 
in special situations, but he recapitulates the species that he found on the 
several different trees. It is not possible to reproduce such a detailed survey, 
which indeed only emphasizes the fact that the physical characters of the 
bark are the most important factors in lichen ecology: that on smooth bark, 

1 Fee 1824. 2 Fries 1831. 3 Krempelhuber 1861. 4 Arnold 1891, etc. 



LICHEN COMMUNITIES 365 

whether of young trees, or on bark that never becomes really rugged, there 
is a preponderance of species with a semi-immersed thallus, and very 
generally of those that are associated with Trentepohlia gonidia, such as 
Graphidaceae or Pyrenulaceae, though certain species of Lecidea, Lecanora 
and others also prefer the smooth substratum. 

Bruce Fink 1 has published a series of important papers on lichen com- 
munities in America, some of them similar to what we should find in the 
British Isles. 

On trees with smooth bark he records in the Minnesota district: 

Xanthoria polycarpa. 
Candelaria concolor. 
Parmelia olivacea, P. adglutinata. 
Placodium cerinum. 
Lecanora subfusca. 
Bacidia fusca-rubella. 
Lecidea enteroleuca. 
Graphis scripta. 

Arlhonia lecideella, A. dispersa. 
Arthopyrenia punctiformis, A.fallax. 
Pyrenula nitida, P. thelena, P. cinerella, P. leucoplaca. 
On rough bark he records : 
Ramalina calicaris, R. fraxinea, R . fastigiata. 

Teloschistes chrysophthalmus. 
Xanthoria polycarpa, X. lychnea. 

Candelaria concolor. 

Parmelia perforata, P. crinita, P. Borreri, P. tiliacea, P. saxatilis, P. caperata. 

Physcia granulifera, Ph. pul-verulenta, Ph. stellaris, Ph. tnbacia, Ph. obscura. 

Collema pycnocarpum, C. flaccidum. 

Leptogium mycochroum. 

Placodium aurantiacum, PL cerinum. 

Lecanora subfusca. 

Perlusaria leioplaca, P. velata. 

Bacidia rubella, B . fuscorubella. 

Leddea enteroleuca. 

Rhizocarpon alboatrum, Buellia parasema. 

Opegrapha varia. 

Graphis scrip ta. 

Arthonia lecideella, A. radiata. 

A>'thopyrenia quinqueseplata, A. macrospora. 

Pyrenula nitida, P. leucoplaca. 

Finally, as generally representative of the commonest lichens in our 
woods of deciduous trees, including both smooth- and rough-barked, the com- 
munity of oak-hazel woods as observed by Watson 2 in Somerset maybequoted: 

Collema flaccidum. 
Calicium hyperellutn. 

i Fink 1902. * Watson 1909. 



366 ECOLOGY 

Ramalina calicaris, R. fraxinea with var. ampliata, R. fastigiata, R. farinacea and 

R. pollinaria. 

Parmelia saxatilis and f. furfuracea, P. caperata, P. physodes. 
Physcia pulverulenta, Ph. tenella (hispida). 
Lecanora subfusca, L. rugosa. 

Pertusaria amara, P. globulifera, P, communis, P. Wuljenii. 
Lecidea (Buellia} canescens. 
Graphis scripta. 

And on the soil of these woods : 

Cladonia pyxidata, Cl. pungens, Cl. macilenta, Cl, pityrea, Cl. squamosa and Cl. 
sylvatica. 

Paulson 1 , from his observations of lichens in Hertfordshire, has concluded 
that the presence or absence of lichens on trees is influenced to a consider- 
able degree by the nature of the soil. They were more abundant in woods 
on light well-drained soils than on similar communities of trees on heavier soils, 
though the shade in the former was slightly more dense and therefore less 
favourable to their development; the cause of this connection is not known. 

c. LlGNICOLOUS. Lichens frequenting the branches of trees do not long 
continue when these have fallen to the ground. This may be due to the 
lack of light and air, but Bouly de Lesdain 2 has suggested that the chemical 
reactions produced by the decomposition of the bast fibres are fatal to them, 
Lecidea parasema alone continuing to grow and even existing for some time 
on the detached shreds of bark. 

On worked wood, such as old doors or old palings, light and air are well 
provided and there is often an abundant growth of lichens, many of which 
seem to prefer that substratum : the fibres of the wood loosened by weathering 
retain moisture and yield some nutriment to the lichen hyphae which burrow 
among them. Though a number of lichens grow willingly on dead wood, 
there are probably none that are wholly restricted to such a habitat. A few, 
such as the species of Coniocybe, are generally to be found on dead roots of 
trees or creeping loosely over dead twigs. They are shade lichens and fond 
of moisture. 

The species on palings or " dead wood communities " most familiar 
to us in our country are : 

Usnea hirta. Rinodina exigua. 

Cetraria diffusa. Lecanora ff agent, L. varia and its allies. 

Evernia furfuracea. Lecidea osfreata, L. parasema. 

Parmelia scortia, P. physodes. Buellia myriocarpa. 

Xanlhoria parietina. Cladoniaceae and Caliciaceae (several species). 

Placodium cerinum. 

These may be found in very varying association. It has indeed been 
remarked that the dominant plant may be simply -the one that has first 
1 Paulson 19 r9- 2 Lesdain 1912. 



LICHEN COMMUNITIES 367 

gained a footing, though the larger and more vigorous lichens tend to crowd 
out the others. Bruce Fink 1 has recorded associations in Minnesota : 
On wood : 

Teloschistes chrysophthalmus. Buellia parasema (disciformis\ B. turgescens. 

Placodium cerinum. Calicium parietinum. 

Lecanora Hagem, L. varia. Thelocarpon prasinellum. 

Rinodina sophodes, R. exigua. 

On rotten stumps and prostrate logs : Peltigera canina. Cladonia fim- 
briata var. tubaeformis, Cl. gracilis, Cl. verticillata. CL symphicarpia, Cl. 
macilenta, Cl. cristatella. 

Except for one or two species such as Buellia turgescens, Cladonia sym- 
phicarpia, etc., the associations could be easy paralleled in our own country, 
though with us Peltigera canina, Cladonia gracilis and Cl. verticillata are 
ground forms. 

2. TERRICOLOUS 

In this community other vegetation is dominant, lichens are subsidiary. 
In certain conditions, as on heaths, they gain a permanent footing, in others 
they are temporary denizens and are easily crowded out. As they are 
generally in close contact with the ground they are peculiarly dependent 
on the nature of the soil and the water content. There are several distinct 
substrata to be considered each with its characteristic flora. Cultivated soil 
and grass lands need scarcely be included, as in the former the processes of 
cultivation are too harassing for lichen growth, and only on the more perma- 
ment somewhat damp mossy meadows do we get such a species as Peltigera 
canina in abundance. Some of the earth-lichens are among the quickest 
growers : the apothecia of Baeomyces rosens appear and disappear within a 
year. Thrombium epigaeum develops in half a year; Thelidium mtnutulum 
in cultures grew from spore to spore, according to Stahl 2 , in three months. 

There are three principal types of soil composition: (i) that in which 
there is more or less of lime; (2) soils in which silica in some form or other 
predominates, and (3) soils which contain an appreciable amount of humus. 

Communities restricted to certain soils such as sand-dunes, etc., are 
treated separately. 

a. ON CALCAREOUS SOIL. Any admixture of lime in the soil, either as 
chalk, limy clay or shell sand is at once reflected in the character of the 
lichen flora. On calcareous soil we may look for any of the squamulose 
Lecanorae or Lecideae that are terricolous species, such as Lecanora crassa, 
L. lentigera, Placodium fulgens, Lecidea lurida and L. decipiens. There are 
also the many lichens that grow on mortar or on the accumulated debris 
mixed with lime in the crevices of walls, such as Biatorina coeruleonigricans, 
species of Placodium, several species of Collema and of Verrucariaceae. 
1 Fink 1896, etc. 3 Stahl 1877. 



368 ECOLOGY 

Bruce Fink 1 found in N.W. Minnesota an association on exposed cal- 
careous earth as follows : 

Heppia Despreauxii. Biatora (Bacidia] muscorum. 

Urceolaria scruposa. Dermatocarpon hepaticum. 

Biatora (Lecidea} decipiens. 

This particular association occupied the slope of a hill that was washed 
by lime-impregnated water. It was normally a dry habitat and the lichens 
were distinguished by small closely adnate thalli. 

There are more lichens confined to limy than to sandy soil. Arnold' 2 
gives a list of those he observed near Munich on the former habitat : 

Cladonia sylvatica f. alpestris. Urceolaria scruposa f. argillacea. 

Cladonia squamosa f. subsquamosa. Verrucatia (Thrombium) epigaea. 

Cladonia rangiformis f. foliosa. Lecidea decipiens. 

Cladonia cariosa and f. sympkicarpa. Dermatocarpon cinereum. 

Peltigera canina f. soreumatica. Collema granulatum. 

Solorina spongiosa. Collema tenax. 

Heppia virescens. Leptogium byssinum. 
Lecanora crassa. 

It is interesting to note how many of these lichens specialized as to 
habitat are forms of species that grow in other situations. 

b. ON SILICEOUS SOIL. Lichens are not generally denizens of cultivated 
soil ; a few settle on clay or on sand-banks. Cladonia fimbriata and Cl. 
pyxidata grow frequently in such situations ; others more or less confined to 
sandy or gravelly soil are, in the British Isles : 

Baeomyces roseus. Gongylia -viridis. 

Baeomyces rufus. Dermatocarpon lachneum. 

Baeomyces placophyllus. Dermatocarpon hepaticum. 

Endocarpon spp. Dermatocarpon cinereum. 

These very generally grow in extended societies of one species only. 

In his enumeration of soil-lichens Arnold 2 gives 40 species that grow on 
siliceous soil, as against 57 on calcareous. Many of them occurred on both. 
Those around Munich on siliceous soil only were : 

Cladonia cocci/era. Baeomyces rufus. 

Cladonia agaridformis. Ler.idea gelatinosa. 

Secoliga (Gyalecta) bryophaga. Psorotichia lutophila. 

Mayfield 3 in his account of the Boulder Clay lichen flora of Suffolk found 
only four species that attained to full development on banks and hedgerows. 
These were: Collema pidposum, Cladonia pyxidata, Cl. furcata var. corymbosa 
and Peltigera polydactyla. 

1 Fink 1902, etc. 2 Arnold 1891. s Mayfield 1916. 



LICHEN COMMUNITIES 369 

On bare heaths of gravelly soil in Epping Forest Paulson and Thompson 1 
describe an association of such lichens as : 

Baeotnyces roseus. Cladonia macilenta. 

Baeomyces rufus. Cladonia furcata. 

Pycnothelia papillaria. Cetraria aculeata. 

Cladonia coccifera. Peltigera spuria. 

Lee idea granulosa. 

And on flints in the soil : Lecidca crustnlata and Rhizocarpon confer- 
voides. They found that Peltigera spuria colonized very quickly the burnt 
patches of earth which are of frequent occurrence in Epping Forest, while 
on wet sandy heaths amongst heather they found associated Cladonia syl- 
vatica f. tennis and Cl. finibriata subsp. y^w/<7. 

c. ON BRICKS, ETC. Closely allied with siliceous soil-lichens are those 
that form communities on bricks. As these when built into walls are more 
or less smeared with mortar, a mixture of lime-loving species also arrives. 
Roof tiles are more free from calcareous matter. Lesdain 2 noted that on 
the dunes, though stray bricks were covered by algae, lichens rarely or never 
seemed to gain a footing. 

There are many references in literature to lichens that live on tiles. 
A fairly representative list is given by Lettau 3 of" tegulicolous " species. 

Verrucaria Jiigrescens. Placodium elegans. 

Lecidea coarctata. Placodium inurorum. 

Candelariella iiitellina. Xantlwria parietina. 

Lecanora dispersa. Rhizocarpon alboatrum var. 

Lecanora galactina. Buellia myriocarpa. 

Lecanora Hageni. Lecidea detnissa. 

Lecanora saxicola. Physcia ascendens. 

Parmelia conspersa. Physcia caesia. 

Placodiuni teicholyttim. Physcia obscura. 

Placodium pyraceuni. Physcia sciastrella. 

Placodiuni decipiens. 

Several of these are more or less calcicolous and others are wanderers, 
indifferent to the substratum. Though certain species form communities on 
bricks, tiles, etc., none of them is restricted to such artificial substrata. 

d. ON HUMUS. Lichens are never found on loose humus, but rocks or 
stumps of trees covered with a thin layer of earth and humus are a favourite 
habitat, especially of Cladoniae. One such " formation " is given by Bruce 
Fink 4 from N. Minnesota ; with the exception of Cladonia cristatclla, the 
species are British as well as American : 

Cladonia furcata. Cladonia rangiferina. 

Cladonia crisiatella. Cladonia untialis. 

Cladonia gracilis. Cladonia alpestris. 

Cladonia verticillata. Cladonia turgida. 

1 Paulson and Thompson 1913. 2 Lesdain 1910-. 3 Lettau 1911, 4 Fink 1903. 

S. L. 24 



370 ECOLOGY 

Cladonia cocci/era. Peltigera malacea. 

Cladonia pyxidata. . Peltigera canina. 

Cladonia fimbriata. Peltigera aphthosa. 

e. ON PEATY SOIL. Peat is generally found in most abundance in 
northern and upland regions, and is characteristic of mountain and moor- 
land, though there are great moss-lands, barely above sea-level, even in our 
own country. Such soil is of an acid nature and attracts a special type of 
plant life. The lichens form no inconsiderable part of the flora, the most 
frequent species being members of the Cladoniaceae. 

The principal crustaceous species on bare peaty soil in the British Isles 
are Lecidea uliginosa and L.granulosa. The former is not easily distinguish- 
able from the soil as both thallus and apothecia are brownish black. The 
latter, which is often associated with it, has a lighter coloured thallus and 
apothecia that change from brick-red to dark brown or black ; Wheldon 
and Wilson 1 remarked that after the burning of the heath it was the first 
vegetation to appear and covered large spaces with its grey thallus. Another 
peat species is Icuiadophila ericetorum, but it prefers damper localities than 
the two Lecideae. 

To quote again from Arnold 2 : 24 species were found on turf around 
Munich, 13 of which were Cladoniae, but only four species could be con- 
sidered as exclusively peat-lichens. These were: 

Cladonia Floerkeana. Thelocarpon turficolum. 

Biatora terricola. Geisleria sychnogonioides. 

The last is a very rare lichen in Central Europe and is generally found 
on sandy soil. Arnold considered that near Munich, for various reasons, 
there was a very poor representation of turf-lichens. 

f. ON MOSSES. Very many lichens grow along with or over mosses, 
either on the ground, 'on rocks or on the bark of trees, doubtless owing to 
the moisture accumulated and retained by these plants. Besides Cladoniae 
the commonest " moss " species in the British Isles are Bilimbia sabulosa, 
Bacidia muscormn, Rinodina Conradi, Lecidea sanguineoatra, Pannaria 
brunnea, Psoroma hypnorum and Lecanora tartarea, with species of Collema 
and Leptogium and Diploschistes bryopJiilus. 

Wheldon and Wilson 3 have listed the lichens that they found in Perth- 
shire on subalpine heath lands, on the ground, or on banks amongst mosses: 

Leptogrum spp. Lecidea granule sa. 

Peltigera spp. Lecidea uliginosa. 

Cetraria spp. Lecidea neglecta. 

Parmelia physodes. Bilimbia sabulosa. 

Psoroma hypnorum. Bilimbia ligniaria. 

Lecanora epibryon. Bilimbia melaena. 

Lecanora tartarea. Baeomyces spp. 

Lecidea coarctata. Cladonia spp. 

1 Wheldon and Wilson 1907. 2 Arnold 1892, p. 34. s Wheldon and Wilson 1915. 



LICHEN COMMUNITIES 371 

As already described Lecanora tartarea* spreads freely over the mosses 
of the tundra. Aigret 2 in a study of Cladoniae notes that Cl. pyxidata, var. 
neglecta chooses little cushions of acrocarpous mosses, which are particularly 
well adapted to retain water. CL digitata, CLflabelliformis and some others 
grow on the mosses which cover old logs or the bases of trees. 

g. ON FUNGI. Some of the fungi, such as Polyporei, are long lived, and 
of hard texture. On species of Lensitcs in Lorraine, Kieffer 3 has recorded 
15 different forms, but they are such as naturally grow on wood and can 
scarcely rank as a separate association. 

3. SAXICOLOUS 

Lichens are the dominant plants of this v and the following formations, 
they alone being able to live on bare rock ; only when there has been formed 
a nidus of soil can other plants become established. 

a. CHARACTERS OF MINERAL SUBSTRATA. It has been often observed 
that lichens are influenced not only by the chemical composition of the 
rocks on which they grow but also by the physical structure. Rocks that 
weather quickly are almost entirely bare of lichens : the breaking up of the 
surface giving no time for the formation either of thallus or fruit. Close- 
grained rocks such as quartzite have also a poor lichen flora, the rooting 
hyphae being unable to penetrate and catch hold. Other factors, such as 
incidence of light, and proximity of water, are of importance in determining 
the nature of the flora, even where the rocks are of similar formation. 

b. COLONIZATION ON ROCKS. When a rock surface is laid bare it 
becomes covered in time with lichens, and quite fresh surfaces are taken 
possession of preferably to weathered surfaces 4 . The number of species is 
largest at first and the kind of lichen depends on the flora existing in the 
near neighbourhood. Link 5 , for instance, has stated that Lichen candelarius 
was the first lichen to appear on the rocks he observed, and, if trees were 
growing near, then Lichen parietinus and Lichen tenellus followed soon after. 
After a time the lichens change, the more slow-growing being crowded out 
by the more vigorous. Crustaceous species, according to Malinowski 8 , are 
most subject to this struggle for existence, and certain types from the nature 
of their thallus are more easily displaced than others. Those with a deeply 
cracked areolated thallus become disintegrated in the older central areas by 
repeated swelling and contracting of the areolae as they change from wet 
to dry conditions. Particles of the thallus are thus easily dislodged, and 
bare places are left, which in time are colonized again by the same lichen 
or by some invading species. There may result a bewildering mosaic of 

1 See p. 358. 2 Aigret 1901. 8 Kieffer 1894. 4 Stahlecker 1906. 

8 Link 1795. Malinowski 1911. 

242 



37 2 ECOLOGY 

different thalli and fruits mingling together. Some